    /******************************************************************************
     * sndif.h
     *
     * Unified sound-device I/O interface for Xen guest OSes.
     *
     * Permission is hereby granted, free of charge, to any person obtaining a copy
     * of this software and associated documentation files (the "Software"), to
     * deal in the Software without restriction, including without limitation the
     * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
     * sell copies of the Software, and to permit persons to whom the Software is
     * furnished to do so, subject to the following conditions:
     *
     * The above copyright notice and this permission notice shall be included in
     * all copies or substantial portions of the Software.
     *
     * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
     * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
     * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
     * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
     * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
     * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
     * DEALINGS IN THE SOFTWARE.
     *
     * Copyright (C) 2013-2015 GlobalLogic Inc.
     * Copyright (C) 2016-2017 EPAM Systems Inc.
     *
     * Authors: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
     *          Oleksandr Grytsov <oleksandr_grytsov@epam.com>
     *          Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>
     *          Iurii Konovalenko <iurii.konovalenko@globallogic.com>
     */

    #ifndef __XEN_PUBLIC_IO_SNDIF_H__
    #define __XEN_PUBLIC_IO_SNDIF_H__

    #include "ring.h"
    #include "../grant_table.h"

    /*
     ******************************************************************************
     *                           Protocol version
     ******************************************************************************
     */
    #define XENSND_PROTOCOL_VERSION         2

    /*
     ******************************************************************************
     *                  Feature and Parameter Negotiation
     ******************************************************************************
     *
     * Front->back notifications: when enqueuing a new request, sending a
     * notification can be made conditional on xensnd_req (i.e., the generic
     * hold-off mechanism provided by the ring macros). Backends must set
     * xensnd_req appropriately (e.g., using RING_FINAL_CHECK_FOR_REQUESTS()).
     *
     * Back->front notifications: when enqueuing a new response, sending a
     * notification can be made conditional on xensnd_resp (i.e., the generic
     * hold-off mechanism provided by the ring macros). Frontends must set
     * xensnd_resp appropriately (e.g., using RING_FINAL_CHECK_FOR_RESPONSES()).
     *
     * The two halves of a para-virtual sound card driver utilize nodes within
     * XenStore to communicate capabilities and to negotiate operating parameters.
     * This section enumerates these nodes which reside in the respective front and
     * backend portions of XenStore, following the XenBus convention.
     *
     * All data in XenStore is stored as strings. Nodes specifying numeric
     * values are encoded in decimal. Integer value ranges listed below are
     * expressed as fixed sized integer types capable of storing the conversion
     * of a properly formated node string, without loss of information.
     *
     ******************************************************************************
     *                        Example configuration
     ******************************************************************************
     *
     * Note: depending on the use-case backend can expose more sound cards and
     * PCM devices/streams than the underlying HW physically has by employing
     * SW mixers, configuring virtual sound streams, channels etc.
     *
     * This is an example of backend and frontend configuration:
     *
     *--------------------------------- Backend -----------------------------------
     *
     * /local/domain/0/backend/vsnd/1/0/frontend-id = "1"
     * /local/domain/0/backend/vsnd/1/0/frontend = "/local/domain/1/device/vsnd/0"
     * /local/domain/0/backend/vsnd/1/0/state = "4"
     * /local/domain/0/backend/vsnd/1/0/versions = "1,2"
     *
     *--------------------------------- Frontend ----------------------------------
     *
     * /local/domain/1/device/vsnd/0/backend-id = "0"
     * /local/domain/1/device/vsnd/0/backend = "/local/domain/0/backend/vsnd/1/0"
     * /local/domain/1/device/vsnd/0/state = "4"
     * /local/domain/1/device/vsnd/0/version = "1"
     *
     *----------------------------- Card configuration ----------------------------
     *
     * /local/domain/1/device/vsnd/0/short-name = "Card short name"
     * /local/domain/1/device/vsnd/0/long-name = "Card long name"
     * /local/domain/1/device/vsnd/0/sample-rates = "8000,32000,44100,48000,96000"
     * /local/domain/1/device/vsnd/0/sample-formats = "s8,u8,s16_le,s16_be"
     * /local/domain/1/device/vsnd/0/buffer-size = "262144"
     *
     *------------------------------- PCM device 0 --------------------------------
     *
     * /local/domain/1/device/vsnd/0/0/name = "General analog"
     * /local/domain/1/device/vsnd/0/0/channels-max = "5"
     *
     *----------------------------- Stream 0, playback ----------------------------
     *
     * /local/domain/1/device/vsnd/0/0/0/type = "p"
     * /local/domain/1/device/vsnd/0/0/0/sample-formats = "s8,u8"
     * /local/domain/1/device/vsnd/0/0/0/unique-id = "0"
     *
     * /local/domain/1/device/vsnd/0/0/0/ring-ref = "386"
     * /local/domain/1/device/vsnd/0/0/0/event-channel = "15"
     * /local/domain/1/device/vsnd/0/0/0/evt-ring-ref = "1386"
     * /local/domain/1/device/vsnd/0/0/0/evt-event-channel = "215"
     *
     *------------------------------ Stream 1, capture ----------------------------
     *
     * /local/domain/1/device/vsnd/0/0/1/type = "c"
     * /local/domain/1/device/vsnd/0/0/1/channels-max = "2"
     * /local/domain/1/device/vsnd/0/0/1/unique-id = "1"
     *
     * /local/domain/1/device/vsnd/0/0/1/ring-ref = "384"
     * /local/domain/1/device/vsnd/0/0/1/event-channel = "13"
     * /local/domain/1/device/vsnd/0/0/1/evt-ring-ref = "1384"
     * /local/domain/1/device/vsnd/0/0/1/evt-event-channel = "213"
     *
     *------------------------------- PCM device 1 --------------------------------
     *
     * /local/domain/1/device/vsnd/0/1/name = "HDMI-0"
     * /local/domain/1/device/vsnd/0/1/sample-rates = "8000,32000,44100"
     *
     *------------------------------ Stream 0, capture ----------------------------
     *
     * /local/domain/1/device/vsnd/0/1/0/type = "c"
     * /local/domain/1/device/vsnd/0/1/0/unique-id = "2"
     *
     * /local/domain/1/device/vsnd/0/1/0/ring-ref = "387"
     * /local/domain/1/device/vsnd/0/1/0/event-channel = "151"
     * /local/domain/1/device/vsnd/0/1/0/evt-ring-ref = "1387"
     * /local/domain/1/device/vsnd/0/1/0/evt-event-channel = "351"
     *
     *------------------------------- PCM device 2 --------------------------------
     *
     * /local/domain/1/device/vsnd/0/2/name = "SPDIF"
     *
     *----------------------------- Stream 0, playback ----------------------------
     *
     * /local/domain/1/device/vsnd/0/2/0/type = "p"
     * /local/domain/1/device/vsnd/0/2/0/unique-id = "3"
     *
     * /local/domain/1/device/vsnd/0/2/0/ring-ref = "389"
     * /local/domain/1/device/vsnd/0/2/0/event-channel = "152"
     * /local/domain/1/device/vsnd/0/2/0/evt-ring-ref = "1389"
     * /local/domain/1/device/vsnd/0/2/0/evt-event-channel = "452"
     *
     ******************************************************************************
     *                            Backend XenBus Nodes
     ******************************************************************************
     *
     *----------------------------- Protocol version ------------------------------
     *
     * versions
     *      Values:         <string>
     *
     *      List of XENSND_LIST_SEPARATOR separated protocol versions supported
     *      by the backend. For example "1,2,3".
     *
     ******************************************************************************
     *                            Frontend XenBus Nodes
     ******************************************************************************
     *
     *-------------------------------- Addressing ---------------------------------
     *
     * dom-id
     *      Values:         <uint16_t>
     *
     *      Domain identifier.
     *
     * dev-id
     *      Values:         <uint16_t>
     *
     *      Device identifier.
     *
     * pcm-dev-idx
     *      Values:         <uint8_t>
     *
     *      Zero based contigous index of the PCM device.
     *
     * stream-idx
     *      Values:         <uint8_t>
     *
     *      Zero based contigous index of the stream of the PCM device.
     *
     * The following pattern is used for addressing:
     *   /local/domain/<dom-id>/device/vsnd/<dev-id>/<pcm-dev-idx>/<stream-idx>/...
     *
     *----------------------------- Protocol version ------------------------------
     *
     * version
     *      Values:         <string>
     *
     *      Protocol version, chosen among the ones supported by the backend.
     *
     *------------------------------- PCM settings --------------------------------
     *
     * Every virtualized sound frontend has a set of PCM devices and streams, each
     * could be individually configured. Part of the PCM configuration can be
     * defined at higher level of the hierarchy and be fully or partially re-used
     * by the underlying layers. These configuration values are:
     *  o number of channels (min/max)
     *  o supported sample rates
     *  o supported sample formats.
     * E.g. one can define these values for the whole card, device or stream.
     * Every underlying layer in turn can re-define some or all of them to better
     * fit its needs. For example, card may define number of channels to be
     * in [1; 8] range, and some particular stream may be limited to [1; 2] only.
     * The rule is that the underlying layer must be a subset of the upper layer
     * range.
     *
     * channels-min
     *      Values:         <uint8_t>
     *
     *      The minimum amount of channels that is supported, [1; channels-max].
     *      Optional, if not set or omitted a value of 1 is used.
     *
     * channels-max
     *      Values:         <uint8_t>
     *
     *      The maximum amount of channels that is supported.
     *      Must be at least <channels-min>.
     *
     * sample-rates
     *      Values:         <list of uint32_t>
     *
     *      List of supported sample rates separated by XENSND_LIST_SEPARATOR.
     *      Sample rates are expressed as a list of decimal values w/o any
     *      ordering requirement.
     *
     * sample-formats
     *      Values:         <list of XENSND_PCM_FORMAT_XXX_STR>
     *
     *      List of supported sample formats separated by XENSND_LIST_SEPARATOR.
     *      Items must not exceed XENSND_SAMPLE_FORMAT_MAX_LEN length.
     *
     * buffer-size
     *      Values:         <uint32_t>
     *
     *      The maximum size in octets of the buffer to allocate per stream.
     *
     *----------------------- Virtual sound card settings -------------------------
     * short-name
     *      Values:         <char[32]>
     *
     *      Short name of the virtual sound card. Optional.
     *
     * long-name
     *      Values:         <char[80]>
     *
     *      Long name of the virtual sound card. Optional.
     *
     *----------------------------- Device settings -------------------------------
     * name
     *      Values:         <char[80]>
     *
     *      Name of the sound device within the virtual sound card. Optional.
     *
     *----------------------------- Stream settings -------------------------------
     *
     * type
     *      Values:         "p", "c"
     *
     *      Stream type: "p" - playback stream, "c" - capture stream
     *
     *      If both capture and playback are needed then two streams need to be
     *      defined under the same device.
     *
     * unique-id
     *      Values:         <string>
     *
     *      After stream initialization it is assigned a unique ID, so every
     *      stream of the frontend can be identified by the backend by this ID.
     *      This can be UUID or such.
     *
     *-------------------- Stream Request Transport Parameters --------------------
     *
     * event-channel
     *      Values:         <uint32_t>
     *
     *      The identifier of the Xen event channel used to signal activity
     *      in the ring buffer.
     *
     * ring-ref
     *      Values:         <uint32_t>
     *
     *      The Xen grant reference granting permission for the backend to map
     *      a sole page in a single page sized ring buffer.
     *
     *--------------------- Stream Event Transport Parameters ---------------------
     *
     * This communication path is used to deliver asynchronous events from backend
     * to frontend, set up per stream.
     *
     * evt-event-channel
     *      Values:         <uint32_t>
     *
     *      The identifier of the Xen event channel used to signal activity
     *      in the ring buffer.
     *
     * evt-ring-ref
     *      Values:         <uint32_t>
     *
     *      The Xen grant reference granting permission for the backend to map
     *      a sole page in a single page sized ring buffer.
     *
     ******************************************************************************
     *                               STATE DIAGRAMS
     ******************************************************************************
     *
     * Tool stack creates front and back state nodes with initial state
     * XenbusStateInitialising.
     * Tool stack creates and sets up frontend sound configuration nodes per domain.
     *
     * Front                                Back
     * =================================    =====================================
     * XenbusStateInitialising              XenbusStateInitialising
     *                                       o Query backend device identification
     *                                         data.
     *                                       o Open and validate backend device.
     *                                                      |
     *                                                      |
     *                                                      V
     *                                      XenbusStateInitWait
     *
     * o Query frontend configuration
     * o Allocate and initialize
     *   event channels per configured
     *   playback/capture stream.
     * o Publish transport parameters
     *   that will be in effect during
     *   this connection.
     *              |
     *              |
     *              V
     * XenbusStateInitialised
     *
     *                                       o Query frontend transport parameters.
     *                                       o Connect to the event channels.
     *                                                      |
     *                                                      |
     *                                                      V
     *                                      XenbusStateConnected
     *
     *  o Create and initialize OS
     *    virtual sound device instances
     *    as per configuration.
     *              |
     *              |
     *              V
     * XenbusStateConnected
     *
     *                                      XenbusStateUnknown
     *                                      XenbusStateClosed
     *                                      XenbusStateClosing
     * o Remove virtual sound device
     * o Remove event channels
     *              |
     *              |
     *              V
     * XenbusStateClosed
     *
     *------------------------------- Recovery flow -------------------------------
     *
     * In case of frontend unrecoverable errors backend handles that as
     * if frontend goes into the XenbusStateClosed state.
     *
     * In case of backend unrecoverable errors frontend tries removing
     * the virtualized device. If this is possible at the moment of error,
     * then frontend goes into the XenbusStateInitialising state and is ready for
     * new connection with backend. If the virtualized device is still in use and
     * cannot be removed, then frontend goes into the XenbusStateReconfiguring state
     * until either the virtualized device removed or backend initiates a new
     * connection. On the virtualized device removal frontend goes into the
     * XenbusStateInitialising state.
     *
     * Note on XenbusStateReconfiguring state of the frontend: if backend has
     * unrecoverable errors then frontend cannot send requests to the backend
     * and thus cannot provide functionality of the virtualized device anymore.
     * After backend is back to normal the virtualized device may still hold some
     * state: configuration in use, allocated buffers, client application state etc.
     * So, in most cases, this will require frontend to implement complex recovery
     * reconnect logic. Instead, by going into XenbusStateReconfiguring state,
     * frontend will make sure no new clients of the virtualized device are
     * accepted, allow existing client(s) to exit gracefully by signaling error
     * state etc.
     * Once all the clients are gone frontend can reinitialize the virtualized
     * device and get into XenbusStateInitialising state again signaling the
     * backend that a new connection can be made.
     *
     * There are multiple conditions possible under which frontend will go from
     * XenbusStateReconfiguring into XenbusStateInitialising, some of them are OS
     * specific. For example:
     * 1. The underlying OS framework may provide callbacks to signal that the last
     *    client of the virtualized device has gone and the device can be removed
     * 2. Frontend can schedule a deferred work (timer/tasklet/workqueue)
     *    to periodically check if this is the right time to re-try removal of
     *    the virtualized device.
     * 3. By any other means.
     *
     ******************************************************************************
     *                             PCM FORMATS
     ******************************************************************************
     *
     * XENSND_PCM_FORMAT_<format>[_<endian>]
     *
     * format: <S/U/F><bits> or <name>
     *     S - signed, U - unsigned, F - float
     *     bits - 8, 16, 24, 32
     *     name - MU_LAW, GSM, etc.
     *
     * endian: <LE/BE>, may be absent
     *     LE - Little endian, BE - Big endian
     */
    #define XENSND_PCM_FORMAT_S8            0
    #define XENSND_PCM_FORMAT_U8            1
    #define XENSND_PCM_FORMAT_S16_LE        2
    #define XENSND_PCM_FORMAT_S16_BE        3
    #define XENSND_PCM_FORMAT_U16_LE        4
    #define XENSND_PCM_FORMAT_U16_BE        5
    #define XENSND_PCM_FORMAT_S24_LE        6
    #define XENSND_PCM_FORMAT_S24_BE        7
    #define XENSND_PCM_FORMAT_U24_LE        8
    #define XENSND_PCM_FORMAT_U24_BE        9
    #define XENSND_PCM_FORMAT_S32_LE        10
    #define XENSND_PCM_FORMAT_S32_BE        11
    #define XENSND_PCM_FORMAT_U32_LE        12
    #define XENSND_PCM_FORMAT_U32_BE        13
    #define XENSND_PCM_FORMAT_F32_LE        14 /* 4-byte float, IEEE-754 32-bit, */
    #define XENSND_PCM_FORMAT_F32_BE        15 /* range -1.0 to 1.0              */
    #define XENSND_PCM_FORMAT_F64_LE        16 /* 8-byte float, IEEE-754 64-bit, */
    #define XENSND_PCM_FORMAT_F64_BE        17 /* range -1.0 to 1.0              */
    #define XENSND_PCM_FORMAT_IEC958_SUBFRAME_LE 18
    #define XENSND_PCM_FORMAT_IEC958_SUBFRAME_BE 19
    #define XENSND_PCM_FORMAT_MU_LAW        20
    #define XENSND_PCM_FORMAT_A_LAW         21
    #define XENSND_PCM_FORMAT_IMA_ADPCM     22
    #define XENSND_PCM_FORMAT_MPEG          23
    #define XENSND_PCM_FORMAT_GSM           24

    /*
     ******************************************************************************
     *                             REQUEST CODES
     ******************************************************************************
     */
    #define XENSND_OP_OPEN                  0
    #define XENSND_OP_CLOSE                 1
    #define XENSND_OP_READ                  2
    #define XENSND_OP_WRITE                 3
    #define XENSND_OP_SET_VOLUME            4
    #define XENSND_OP_GET_VOLUME            5
    #define XENSND_OP_MUTE                  6
    #define XENSND_OP_UNMUTE                7
    #define XENSND_OP_TRIGGER               8
    #define XENSND_OP_HW_PARAM_QUERY        9

    #define XENSND_OP_TRIGGER_START         0
    #define XENSND_OP_TRIGGER_PAUSE         1
    #define XENSND_OP_TRIGGER_STOP          2
    #define XENSND_OP_TRIGGER_RESUME        3

    /*
     ******************************************************************************
     *                                 EVENT CODES
     ******************************************************************************
     */
    #define XENSND_EVT_CUR_POS              0

    /*
     ******************************************************************************
     *               XENSTORE FIELD AND PATH NAME STRINGS, HELPERS
     ******************************************************************************
     */
    #define XENSND_DRIVER_NAME              "vsnd"

    #define XENSND_LIST_SEPARATOR           ","
    /* Field names */
    #define XENSND_FIELD_BE_VERSIONS        "versions"
    #define XENSND_FIELD_FE_VERSION         "version"
    #define XENSND_FIELD_VCARD_SHORT_NAME   "short-name"
    #define XENSND_FIELD_VCARD_LONG_NAME    "long-name"
    #define XENSND_FIELD_RING_REF           "ring-ref"
    #define XENSND_FIELD_EVT_CHNL           "event-channel"
    #define XENSND_FIELD_EVT_RING_REF       "evt-ring-ref"
    #define XENSND_FIELD_EVT_EVT_CHNL       "evt-event-channel"
    #define XENSND_FIELD_DEVICE_NAME        "name"
    #define XENSND_FIELD_TYPE               "type"
    #define XENSND_FIELD_STREAM_UNIQUE_ID   "unique-id"
    #define XENSND_FIELD_CHANNELS_MIN       "channels-min"
    #define XENSND_FIELD_CHANNELS_MAX       "channels-max"
    #define XENSND_FIELD_SAMPLE_RATES       "sample-rates"
    #define XENSND_FIELD_SAMPLE_FORMATS     "sample-formats"
    #define XENSND_FIELD_BUFFER_SIZE        "buffer-size"

    /* Stream type field values. */
    #define XENSND_STREAM_TYPE_PLAYBACK     "p"
    #define XENSND_STREAM_TYPE_CAPTURE      "c"
    /* Sample rate max string length */
    #define XENSND_SAMPLE_RATE_MAX_LEN      11
    /* Sample format field values */
    #define XENSND_SAMPLE_FORMAT_MAX_LEN    24

    #define XENSND_PCM_FORMAT_S8_STR        "s8"
    #define XENSND_PCM_FORMAT_U8_STR        "u8"
    #define XENSND_PCM_FORMAT_S16_LE_STR    "s16_le"
    #define XENSND_PCM_FORMAT_S16_BE_STR    "s16_be"
    #define XENSND_PCM_FORMAT_U16_LE_STR    "u16_le"
    #define XENSND_PCM_FORMAT_U16_BE_STR    "u16_be"
    #define XENSND_PCM_FORMAT_S24_LE_STR    "s24_le"
    #define XENSND_PCM_FORMAT_S24_BE_STR    "s24_be"
    #define XENSND_PCM_FORMAT_U24_LE_STR    "u24_le"
    #define XENSND_PCM_FORMAT_U24_BE_STR    "u24_be"
    #define XENSND_PCM_FORMAT_S32_LE_STR    "s32_le"
    #define XENSND_PCM_FORMAT_S32_BE_STR    "s32_be"
    #define XENSND_PCM_FORMAT_U32_LE_STR    "u32_le"
    #define XENSND_PCM_FORMAT_U32_BE_STR    "u32_be"
    #define XENSND_PCM_FORMAT_F32_LE_STR    "float_le"
    #define XENSND_PCM_FORMAT_F32_BE_STR    "float_be"
    #define XENSND_PCM_FORMAT_F64_LE_STR    "float64_le"
    #define XENSND_PCM_FORMAT_F64_BE_STR    "float64_be"
    #define XENSND_PCM_FORMAT_IEC958_SUBFRAME_LE_STR "iec958_subframe_le"
    #define XENSND_PCM_FORMAT_IEC958_SUBFRAME_BE_STR "iec958_subframe_be"
    #define XENSND_PCM_FORMAT_MU_LAW_STR    "mu_law"
    #define XENSND_PCM_FORMAT_A_LAW_STR     "a_law"
    #define XENSND_PCM_FORMAT_IMA_ADPCM_STR "ima_adpcm"
    #define XENSND_PCM_FORMAT_MPEG_STR      "mpeg"
    #define XENSND_PCM_FORMAT_GSM_STR       "gsm"

    /*
     ******************************************************************************
     *                          STATUS RETURN CODES
     ******************************************************************************
     *
     * Status return code is zero on success and -XEN_EXX on failure.
     *
     ******************************************************************************
     *                              Assumptions
     ******************************************************************************
     * o usage of grant reference 0 as invalid grant reference:
     *   grant reference 0 is valid, but never exposed to a PV driver,
     *   because of the fact it is already in use/reserved by the PV console.
     * o all references in this document to page sizes must be treated
     *   as pages of size XEN_PAGE_SIZE unless otherwise noted.
     *
     ******************************************************************************
     *       Description of the protocol between frontend and backend driver
     ******************************************************************************
     *
     * The two halves of a Para-virtual sound driver communicate with
     * each other using shared pages and event channels.
     * Shared page contains a ring with request/response packets.
     *
     * Packets, used for input/output operations, e.g. read/write, set/get volume,
     * etc., provide offset/length fields in order to allow asynchronous protocol
     * operation with buffer space sharing: part of the buffer allocated at
     * XENSND_OP_OPEN can be used for audio samples and part, for example,
     * for volume control.
     *
     * All reserved fields in the structures below must be 0.
     *
     *---------------------------------- Requests ---------------------------------
     *
     * All request packets have the same length (64 octets)
     * All request packets have common header:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |    operation   |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     *   id - uint16_t, private guest value, echoed in response
     *   operation - uint8_t, operation code, XENSND_OP_???
     *
     * For all packets which use offset and length:
     *   offset - uint32_t, read or write data offset within the shared buffer,
     *     passed with XENSND_OP_OPEN request, octets,
     *     [0; XENSND_OP_OPEN.buffer_sz - 1].
     *   length - uint32_t, read or write data length, octets
     *
     * Request open - open a PCM stream for playback or capture:
     *
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                | XENSND_OP_OPEN |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                             pcm_rate                              | 12
     * +----------------+----------------+----------------+----------------+
     * |  pcm_format    |  pcm_channels  |             reserved            | 16
     * +----------------+----------------+----------------+----------------+
     * |                             buffer_sz                             | 20
     * +----------------+----------------+----------------+----------------+
     * |                           gref_directory                          | 24
     * +----------------+----------------+----------------+----------------+
     * |                             period_sz                             | 28
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 32
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * pcm_rate - uint32_t, stream data rate, Hz
     * pcm_format - uint8_t, XENSND_PCM_FORMAT_XXX value
     * pcm_channels - uint8_t, number of channels of this stream,
     *   [channels-min; channels-max]
     * buffer_sz - uint32_t, buffer size to be allocated, octets
     * period_sz - uint32_t, event period size, octets
     *   This is the requested value of the period at which frontend would
     *   like to receive XENSND_EVT_CUR_POS notifications from the backend when
     *   stream position advances during playback/capture.
     *   It shows how many octets are expected to be played/captured before
     *   sending such an event.
     *   If set to 0 no XENSND_EVT_CUR_POS events are sent by the backend.
     *
     * gref_directory - grant_ref_t, a reference to the first shared page
     *   describing shared buffer references. At least one page exists. If shared
     *   buffer size  (buffer_sz) exceeds what can be addressed by this single page,
     *   then reference to the next page must be supplied (see gref_dir_next_page
     *   below)
     */

    struct xensnd_open_req {
        uint32_t pcm_rate;
        uint8_t pcm_format;
        uint8_t pcm_channels;
        uint16_t reserved;
        uint32_t buffer_sz;
        grant_ref_t gref_directory;
        uint32_t period_sz;
    };

    /*
     * Shared page for XENSND_OP_OPEN buffer descriptor (gref_directory in the
     *   request) employs a list of pages, describing all pages of the shared data
     *   buffer:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |                        gref_dir_next_page                         | 4
     * +----------------+----------------+----------------+----------------+
     * |                              gref[0]                              | 8
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                              gref[i]                              | i*4+8
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             gref[N - 1]                           | N*4+8
     * +----------------+----------------+----------------+----------------+
     *
     * gref_dir_next_page - grant_ref_t, reference to the next page describing
     *   page directory. Must be 0 if there are no more pages in the list.
     * gref[i] - grant_ref_t, reference to a shared page of the buffer
     *   allocated at XENSND_OP_OPEN
     *
     * Number of grant_ref_t entries in the whole page directory is not
     * passed, but instead can be calculated as:
     *   num_grefs_total = (XENSND_OP_OPEN.buffer_sz + XEN_PAGE_SIZE - 1) /
     *       XEN_PAGE_SIZE
     */

    struct xensnd_page_directory {
        grant_ref_t gref_dir_next_page;
        grant_ref_t gref[1]; /* Variable length */
    };

    /*
     *  Request close - close an opened pcm stream:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                | XENSND_OP_CLOSE|    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * Request read/write - used for read (for capture) or write (for playback):
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |   operation    |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                              offset                               | 12
     * +----------------+----------------+----------------+----------------+
     * |                              length                               | 16
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 20
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * operation - XENSND_OP_READ for read or XENSND_OP_WRITE for write
     */

    struct xensnd_rw_req {
        uint32_t offset;
        uint32_t length;
    };

    /*
     * Request set/get volume - set/get channels' volume of the stream given:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |   operation    |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                              offset                               | 12
     * +----------------+----------------+----------------+----------------+
     * |                              length                               | 16
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 20
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * operation - XENSND_OP_SET_VOLUME for volume set
     *   or XENSND_OP_GET_VOLUME for volume get
     * Buffer passed with XENSND_OP_OPEN is used to exchange volume
     * values:
     *
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |                             channel[0]                            | 4
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             channel[i]                            | i*4
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                           channel[N - 1]                          | (N-1)*4
     * +----------------+----------------+----------------+----------------+
     *
     * N = XENSND_OP_OPEN.pcm_channels
     * i - uint8_t, index of a channel
     * channel[i] - sint32_t, volume of i-th channel
     * Volume is expressed as a signed value in steps of 0.001 dB,
     * while 0 being 0 dB.
     *
     * Request mute/unmute - mute/unmute stream:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |   operation    |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                              offset                               | 12
     * +----------------+----------------+----------------+----------------+
     * |                              length                               | 16
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 20
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * operation - XENSND_OP_MUTE for mute or XENSND_OP_UNMUTE for unmute
     * Buffer passed with XENSND_OP_OPEN is used to exchange mute/unmute
     * values:
     *
     *                                   0                                 octet
     * +----------------+----------------+----------------+----------------+
     * |                             channel[0]                            | 4
     * +----------------+----------------+----------------+----------------+
     * +/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             channel[i]                            | i*4
     * +----------------+----------------+----------------+----------------+
     * +/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                           channel[N - 1]                          | (N-1)*4
     * +----------------+----------------+----------------+----------------+
     *
     * N = XENSND_OP_OPEN.pcm_channels
     * i - uint8_t, index of a channel
     * channel[i] - uint8_t, non-zero if i-th channel needs to be muted/unmuted
     *
     *------------------------------------ N.B. -----------------------------------
     *
     * The 'struct xensnd_rw_req' is also used for XENSND_OP_SET_VOLUME,
     * XENSND_OP_GET_VOLUME, XENSND_OP_MUTE, XENSND_OP_UNMUTE.
     *
     * Request stream running state change - trigger PCM stream running state
     * to start, stop, pause or resume:
     *
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |   _OP_TRIGGER  |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |      type      |                     reserved                     | 12
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 16
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * type - uint8_t, XENSND_OP_TRIGGER_XXX value
     */

    struct xensnd_trigger_req {
        uint8_t type;
    };

    /*
     * Request stream parameter ranges: request intervals and
     *   masks of supported ranges for stream configuration values.
     *
     *   Sound device configuration for a particular stream is a limited subset
     *   of the multidimensional configuration available on XenStore, e.g.
     *   once the frame rate has been selected there is a limited supported range
     *   for sample rates becomes available (which might be the same set configured
     *   on XenStore or less). For example, selecting 96kHz sample rate may limit
     *   number of channels available for such configuration from 4 to 2, etc.
     *   Thus, each call to XENSND_OP_HW_PARAM_QUERY may reduce configuration
     *   space making it possible to iteratively get the final stream configuration,
     *   used in XENSND_OP_OPEN request.
     *
     *   See response format for this request.
     *
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                | _HW_PARAM_QUERY|    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                     formats mask low 32-bit                       | 12
     * +----------------+----------------+----------------+----------------+
     * |                     formats mask high 32-bit                      | 16
     * +----------------+----------------+----------------+----------------+
     * |                              min rate                             | 20
     * +----------------+----------------+----------------+----------------+
     * |                              max rate                             | 24
     * +----------------+----------------+----------------+----------------+
     * |                            min channels                           | 28
     * +----------------+----------------+----------------+----------------+
     * |                            max channels                           | 32
     * +----------------+----------------+----------------+----------------+
     * |                         min buffer frames                         | 36
     * +----------------+----------------+----------------+----------------+
     * |                         max buffer frames                         | 40
     * +----------------+----------------+----------------+----------------+
     * |                         min period frames                         | 44
     * +----------------+----------------+----------------+----------------+
     * |                         max period frames                         | 48
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 52
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * formats - uint64_t, bit mask representing values of the parameter
     *   made as bitwise OR of (1 << XENSND_PCM_FORMAT_XXX) values
     *
     * For interval parameters:
     *   min - uint32_t, minimum value of the parameter
     *   max - uint32_t, maximum value of the parameter
     *
     * Frame is defined as a product of the number of channels by the
     * number of octets per one sample.
     */

    struct xensnd_query_hw_param {
        uint64_t formats;
        struct {
            uint32_t min;
            uint32_t max;
        } rates;
        struct {
            uint32_t min;
            uint32_t max;
        } channels;
        struct {
            uint32_t min;
            uint32_t max;
        } buffer;
        struct {
            uint32_t min;
            uint32_t max;
        } period;
    };

    /*
     *---------------------------------- Responses --------------------------------
     *
     * All response packets have the same length (64 octets)
     *
     * All response packets have common header:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |    operation   |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                              status                               | 8
     * +----------------+----------------+----------------+----------------+
     *
     * id - uint16_t, copied from the request
     * operation - uint8_t, XENSND_OP_* - copied from request
     * status - int32_t, response status, zero on success and -XEN_EXX on failure
     *
     *
     * HW parameter query response - response for XENSND_OP_HW_PARAM_QUERY:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |    operation   |    reserved    | 4
     * +----------------+----------------+----------------+----------------+
     * |                              status                               | 8
     * +----------------+----------------+----------------+----------------+
     * |                     formats mask low 32-bit                       | 12
     * +----------------+----------------+----------------+----------------+
     * |                     formats mask high 32-bit                      | 16
     * +----------------+----------------+----------------+----------------+
     * |                              min rate                             | 20
     * +----------------+----------------+----------------+----------------+
     * |                              max rate                             | 24
     * +----------------+----------------+----------------+----------------+
     * |                            min channels                           | 28
     * +----------------+----------------+----------------+----------------+
     * |                            max channels                           | 32
     * +----------------+----------------+----------------+----------------+
     * |                         min buffer frames                         | 36
     * +----------------+----------------+----------------+----------------+
     * |                         max buffer frames                         | 40
     * +----------------+----------------+----------------+----------------+
     * |                         min period frames                         | 44
     * +----------------+----------------+----------------+----------------+
     * |                         max period frames                         | 48
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 52
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * Meaning of the values in this response is the same as for
     * XENSND_OP_HW_PARAM_QUERY request.
     */

    /*
     *----------------------------------- Events ----------------------------------
     *
     * Events are sent via shared page allocated by the front and propagated by
     *   evt-event-channel/evt-ring-ref XenStore entries
     * All event packets have the same length (64 octets)
     * All event packets have common header:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |      type      |   reserved     | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     *
     * id - uint16_t, event id, may be used by front
     * type - uint8_t, type of the event
     *
     *
     * Current stream position - event from back to front when stream's
     *   playback/capture position has advanced:
     *         0                1                 2               3        octet
     * +----------------+----------------+----------------+----------------+
     * |               id                |   _EVT_CUR_POS |   reserved     | 4
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 8
     * +----------------+----------------+----------------+----------------+
     * |                         position low 32-bit                       | 12
     * +----------------+----------------+----------------+----------------+
     * |                         position high 32-bit                      | 16
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 20
     * +----------------+----------------+----------------+----------------+
     * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
     * +----------------+----------------+----------------+----------------+
     * |                             reserved                              | 64
     * +----------------+----------------+----------------+----------------+
     *
     * position - current value of stream's playback/capture position, octets
     *
     */

    struct xensnd_cur_pos_evt {
        uint64_t position;
    };

    struct xensnd_req {
        uint16_t id;
        uint8_t operation;
        uint8_t reserved[5];
        union {
            struct xensnd_open_req open;
            struct xensnd_rw_req rw;
            struct xensnd_trigger_req trigger;
            struct xensnd_query_hw_param hw_param;
            uint8_t reserved[56];
        } op;
    };

    struct xensnd_resp {
        uint16_t id;
        uint8_t operation;
        uint8_t reserved;
        int32_t status;
        union {
            struct xensnd_query_hw_param hw_param;
            uint8_t reserved1[56];
        } resp;
    };

    struct xensnd_evt {
        uint16_t id;
        uint8_t type;
        uint8_t reserved[5];
        union {
            struct xensnd_cur_pos_evt cur_pos;
            uint8_t reserved[56];
        } op;
    };

    DEFINE_RING_TYPES(xen_sndif, struct xensnd_req, struct xensnd_resp);

    /*
     ******************************************************************************
     *                        Back to front events delivery
     ******************************************************************************
     * In order to deliver asynchronous events from back to front a shared page is
     * allocated by front and its granted reference propagated to back via
     * XenStore entries (evt-ring-ref/evt-event-channel).
     * This page has a common header used by both front and back to synchronize
     * access and control event's ring buffer, while back being a producer of the
     * events and front being a consumer. The rest of the page after the header
     * is used for event packets.
     *
     * Upon reception of an event(s) front may confirm its reception
     * for either each event, group of events or none.
     */

    struct xensnd_event_page {
        uint32_t in_cons;
        uint32_t in_prod;
        uint8_t reserved[56];
    };

    #define XENSND_EVENT_PAGE_SIZE 4096
    #define XENSND_IN_RING_OFFS (sizeof(struct xensnd_event_page))
    #define XENSND_IN_RING_SIZE (XENSND_EVENT_PAGE_SIZE - XENSND_IN_RING_OFFS)
    #define XENSND_IN_RING_LEN (XENSND_IN_RING_SIZE / sizeof(struct xensnd_evt))
    #define XENSND_IN_RING(page) \
        ((struct xensnd_evt *)((char *)(page) + XENSND_IN_RING_OFFS))
    #define XENSND_IN_RING_REF(page, idx) \
        (XENSND_IN_RING((page))[(idx) % XENSND_IN_RING_LEN])

    #endif /* __XEN_PUBLIC_IO_SNDIF_H__ */

    /*
     * Local variables:
     * mode: C
     * c-file-style: "BSD"
     * c-basic-offset: 4
     * tab-width: 4
     * indent-tabs-mode: nil
     * End:
     */