sndif: add ABI for para-virtual sound

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+/******************************************************************************
+ * 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 EPAM Systems Inc.
+ *
+ * Authors: Oleksandr Andrushchenko <[email protected]>
+ *          Oleksandr Grytsov <[email protected]>
+ *          Oleksandr Dmytryshyn <[email protected]>
+ *          Iurii Konovalenko <[email protected]>
+ */
+
+#ifndef __XEN_PUBLIC_IO_XENSND_H__
+#define __XEN_PUBLIC_IO_XENSND_H__
+
+#include "ring.h"
+#include "../grant_table.h"
+
+/*
+ * Front->back notifications: When enqueuing a new request, sending a
+ * notification can be made conditional on req_event (i.e., the generic
+ * hold-off mechanism provided by the ring macros). Backends must set
+ * req_event 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 rsp_event (i.e., the generic
+ * hold-off mechanism provided by the ring macros). Frontends must set
+ * rsp_event appropriately (e.g., using RING_FINAL_CHECK_FOR_RESPONSES()).
+ */
+
+/*
+ * Feature and Parameter Negotiation
+ * =================================
+ * The two halves of a Para-virtual sound card driver utilize nodes within the
+ * XenStore to communicate capabilities and to negotiate operating parameters.
+ * This section enumerates these nodes which reside in the respective front and
+ * backend portions of the XenStore, following the XenBus convention.
+ *
+ * All data in the 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.
+ *
+ *****************************************************************************
+ *                            Backend XenBus Nodes
+ *****************************************************************************
+ *
+ *-------------------------------- Addressing ---------------------------------
+ *
+ * Indices used to address frontends, driver instances, cards,
+ * devices and streams.
+ *
+ * frontend_id
+ *      Values:         <uint>
+ *
+ *      Domain ID of the sound frontend.
+ *
+ * drv_idx
+ *      Values:         <uint>
+ *
+ *      Zero based contiguous index of the virtualized sound driver instance in
+ *      this domain. Multiple PV drivers are allowed in the domain
+ *      at the same time.
+ *
+ * dev_id
+ *      Values:         <uint>
+ *
+ *      Unique device ID.
+ *      Doesn't have to be zero based and/or to be contiguous.
+ *
+ * stream_idx
+ *      Values:         <uint>
+ *
+ *      Zero based contiguous index of the stream of the device.
+ *
+ * Example for the frontend running in domain 5, instance of the driver
+ * in the front is 0 (single or first PV driver), device id 2,
+ * first stream (0):
+ * /local/domain/<frontend_id>/device/vsnd/<drv_idx>/
+ *         device/<dev_id>/stream/<stream_idx>/type = "p"
+ * /local/domain/5/device/vsnd/0/device/2/stream/0/type = "p"
+ *
+ *------------------------------- PCM settings --------------------------------
+ *
+ * Every virtualized sound frontend has set of devices and streams, each
+ * is individually configured. Part of the PCM configuration can be defined at
+ * higher level 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 driver, device or stream.
+ * Every underlying layer in turn can re-define some or all of them to better
+ * fit its needs. For example, driver 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.
+ *
+ * Note: if any of the values are not defined then PV driver should use
+ * its default values instead.
+ *
+ * channels-min
+ *      Values:         <uint>
+ *
+ *      The minimum amount of channels that is supported.
+ *      Must be at least 1. If not defined then use frontend's default.
+ *
+ * channels-max
+ *      Values:         <uint>
+ *
+ *      The maximum amount of channels that is supported.
+ *      Must be at least <channels-min>. If not defined then use frontend's
+ *      default.
+ *
+ * sample-rates
+ *      Values:         <list of uints>
+ *
+ *      List of supported sample rates separated by XENSND_LIST_SEPARATOR.
+ *      If not defined then use frontend's default. 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.
+ *      If not defined then use frontend's default.
+ *
+ * buffer-size
+ *      Values:         <uint>
+ *
+ *      The maximum size in octets of the buffer to allocate per stream.
+ *
+ * Example configuration:
+ *
+ * Driver configuration used by all streams:
+ * /local/domain/5/device/vsnd/0/sample-formats = "s8;u8;s16_le;s16_be"
+ * Stream overrides sample rates supported:
+ * /local/domain/5/device/vsnd/0/device/2/stream/0/sample-rates =
+ *        "8000;22050;44100;48000"
+ *
+ *----------------------- 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.
+ *
+ * For example,
+ * /local/domain/5/device/vsnd/0/short-name = "Virtual audio"
+ * /local/domain/5/device/vsnd/0/long-name =
+ *         "Virtual audio at center stack"
+ *
+ *----------------------------- Device settings --------------------------------
+ * name
+ *      Values:         <char[80]>
+ *
+ *      Name of the sound device within the virtual sound card. Optional.
+ *
+ * For example,
+ * /local/domain/5/device/vsnd/0/device/0/name = "General analog"
+ *
+ *----------------------------- 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. For example,
+ *      /local/domain/5/device/vsnd/0/device/0/stream/0/type = "p"
+ *      /local/domain/5/device/vsnd/0/device/0/stream/1/type = "c"
+ *
+ *****************************************************************************
+ *                            Frontend XenBus Nodes
+ *****************************************************************************
+ *
+ *----------------------- Request Transport Parameters -----------------------
+ *
+ * These are per stream.
+ *
+ * event-channel
+ *      Values:         <uint>
+ *
+ *      The identifier of the Xen event channel used to signal activity
+ *      in the ring buffer.
+ *
+ * ring-ref
+ *      Values:         <uint>
+ *
+ *      The Xen grant reference granting permission for the backend to map
+ *      a sole page in a single page sized ring buffer.
+ *
+ * index
+ *      Values:         <uint>
+ *
+ *      After stream initialization it is assigned a unique ID (within the front
+ *      driver), so every stream of the frontend can be identified by the
+ *      backend by this ID. This is not equal to stream_idx as the later is
+ *      zero based within a device, but this index is contiguous within the
+ *      driver.
+ */
+
+/*
+ * STATE DIAGRAMS
+ *
+ *****************************************************************************
+ *                                   Startup                                 *
+ *****************************************************************************
+ *
+ * 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
+ *
+ */
+
+/*
+ * 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
+
+/*
+ * XENSTORE FIELD AND PATH NAME STRINGS, HELPERS.
+ */
+#define XENSND_DRIVER_NAME                   "vsnd"
+
+#define XENSND_LIST_SEPARATOR                ";"
+/* Path entries */
+#define XENSND_PATH_DEVICE                   "device"
+#define XENSND_PATH_STREAM                   "stream"
+/* Field names */
+#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_DEVICE_NAME             "name"
+#define XENSND_FIELD_TYPE                    "type"
+#define XENSND_FIELD_STREAM_INDEX            "index"
+#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            6
+/* 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.
+ */
+ /* Operation failed for some unspecified reason (e. g. -EIO). */
+#define XENSND_RSP_ERROR                 (-1)
+ /* Operation completed successfully. */
+#define XENSND_RSP_OKAY                  0
+
+/*
+ * Description of the protocol between frontend and backend driver.
+ *
+ * The two halves of a Para-virtual sound driver communicates with
+ * each other using a shared page and an event channel.
+ * Shared page contains a ring with request/response packets.
+ *
+ *
+ * All request packets have the same length (64 octets)
+ *
+ *
+ * Request open - open a PCM stream for playback or capture:
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                pcm_rate                               |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |  pcm_format     |  pcm_channels   |             reserved              |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                         gref_directory_start                          |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, private guest value, echoed in response
+ * operation - uint8_t, XENSND_OP_OPEN
+ * stream_idx - uint8_t, index of the stream ("streams_idx" XenStore entry
+ *   of the stream)
+ * 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
+ * gref_directory_start - grant_ref_t, a reference to the first shared page
+ *   describing shared buffer references. At least one page exists. If shared
+ *   buffer size exceeds what can be addressed by this single page, then
+ *   reference to the next page must be supplied (gref_dir_next_page below
+ *   is not NULL)
+ */
+
+struct xensnd_open_req {
+    uint32_t pcm_rate; /* in Hz */
+    uint8_t pcm_format;
+    uint8_t pcm_channels;
+    uint16_t __reserved0;
+    grant_ref_t gref_directory_start;
+};
+
+/*
+ * 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                           |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                num_grefs                              |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                gref[0]                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                gref[1]                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                gref[N -1]                             |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * gref_dir_next_page - grant_ref_t, reference to the next page describing
+ *   page directory
+ * num_grefs - number of grefs in this page
+ * gref[i] - grant_ref_t, reference to a shared page of the buffer
+ *   allocated at XENSND_OP_OPEN
+ */
+
+struct xensnd_page_directory {
+    grant_ref_t gref_dir_next_page;
+    uint32_t num_grefs;
+    grant_ref_t gref[0];
+};
+
+/*
+ *  Request close - close an opened pcm stream:
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, private guest value, echoed in response
+ * operation - uint8_t, XENSND_OP_CLOSE
+ * stream_idx - uint8_t, index of the stream ("streams_idx" XenStore entry
+ *   of the stream)
+ */
+
+struct xensnd_close_req {
+    /* place holder, remove if changing the structure (C89 concern) */
+    uint8_t __placeholder;
+};
+
+/*
+ * Request read/write - used for read (for capture) or write (for playback):
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                offset                                 |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                length                                 |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, private guest value, echoed in response
+ * operation - uint8_t, XENSND_OP_READ/XENSND_OP_WRITE
+ * stream_idx - uint8_t, index of the stream ("streams_idx" XenStore entry
+ *   of the stream)
+ * offset - uint32_t, read or write data offset within the shared buffer
+ *   passed with XENSND_OP_OPEN request
+ * length - uint32_t, read or write data length
+ */
+
+struct xensnd_write_req {
+    uint32_t offset;
+    uint32_t len;
+};
+
+struct xensnd_read_req {
+    uint32_t offset;
+    uint32_t len;
+};
+
+/*
+ * Request set/get volume - set/get channels' volume of the stream given:
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, private guest value, echoed in response
+ * operation - uint8_t, XENSND_OP_SET_VOLUME or XENSND_OP_GET_VOLUME
+ * stream_idx - uint8_t, index of the stream ("streams_idx" XenStore entry
+ *   of the stream)
+ * Buffer passed with XENSND_OP_OPEN is used to exchange volume
+ * values:
+ *
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               channel[0]                              |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               channel[1]                              |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * +/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ *                  channel[XENSND_OP_OPEN.pcm_channels - 1]               |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * 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.
+ */
+
+struct xensnd_get_vol_req {
+    /* place holder, remove if changing the structure (C89 concern) */
+    uint8_t __placeholder;
+};
+
+struct xensnd_set_vol_req {
+    /* place holder, remove if changing the structure (C89 concern) */
+    uint8_t __placeholder;
+};
+
+/*
+ * Request mute/unmute - mute/unmute stream:
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, private guest value, echoed in response
+ * operation - uint8_t, XENSND_OP_MUTE/XENSND_OP_UNMUTE
+ * stream_idx - uint8_t, index of the stream ("streams_idx" XenStore entry
+ *   of the stream)
+ * Buffer passed with XENSND_OP_OPEN is used to exchange mute/unmute
+ * values:
+ *
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |   channel[0]    |   channel[1]    |   channel[2]    |   channel[3]    |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * +/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |   channel[i]    |   channel[i+1]  |   channel[i+2]  |   channel[i+3]  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * +/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * channel[i] - uint8_t, non-zero if i-th channel needs to be muted/unmuted
+ * Number of channels passed is equal to XENSND_OP_OPEN request pcm_channels
+ * field
+ */
+
+struct xensnd_mute_req {
+    /* place holder, remove if changing the structure (C89 concern) */
+    uint8_t __placeholder;
+};
+
+struct xensnd_unmute_req {
+    /* place holder, remove if changing the structure (C89 concern) */
+    uint8_t __placeholder;
+};
+
+/*
+ * All response packets have the same length (64 bytes)
+ *
+ * Response for all requests:
+ *          0                 1                  2                3        octet
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                 id                |    operation    |     stream_idx  |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                                padding                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |      status     |                      reserved                       |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                              reserved                                 |
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +-----------------+-----------------+-----------------+-----------------+
+ * |                               reserved                                |
+ * +-----------------+-----------------+-----------------+-----------------+
+ *
+ * id - uint16_t, copied from the request
+ * stream_idx - uint8_t, copied from request
+ * operation - uint8_t, XENSND_OP_XXX - copied from request
+ * status - int8_t, response status (XENSND_RSP_???)
+ */
+
+union xensnd_req {
+    struct {
+        uint16_t id;
+        uint8_t operation;
+        uint8_t stream_idx;
+        uint32_t padding;
+        union {
+            struct xensnd_open_req open;
+                struct xensnd_close_req close;
+                struct xensnd_write_req write;
+                struct xensnd_read_req read;
+                struct xensnd_get_vol_req get_vol;
+                struct xensnd_set_vol_req set_vol;
+                struct xensnd_mute_req mute;
+                struct xensnd_unmute_req unmute;
+            } op;
+        } data;
+    uint8_t raw[64];
+};
+
+union xensnd_resp {
+    struct {
+        uint16_t id;
+        uint8_t operation;
+        uint8_t stream_idx;
+        int8_t status;
+    } data;
+    uint8_t raw[64];
+};
+
+DEFINE_RING_TYPES(xen_sndif, union xensnd_req, union xensnd_resp);
+
+#endif /* __XEN_PUBLIC_IO_XENSND_H__ */