xen/sndif: add sound-device ABI

Add ABI for the two halves of a para-virtualized
sound driver to communicate with each other.

The ABI allows implementing audio playback and capture as
well as volume control and possibility to mute/unmute
audio sources.

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. Thus, allowing fine tunned configurations per
frontend.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>
Signed-off-by: Iurii Konovalenko <iurii.konovalenko@globallogic.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

include/xen/interface/io/sndif.h

+/******************************************************************************
+ * 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"
+
+/*
+ ******************************************************************************
+ *                  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"
+ *
+ *------------------------------ 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"
+ *
+ *------------------------------- 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"
+ *
+ *------------------------------- 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"
+ *
+ ******************************************************************************
+ *                            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:         <uint32_t>
+ *
+ *      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 the device, but this index is contigous within the
+ *      driver.
+ *
+ *-------------------- 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.
+ *
+ ******************************************************************************
+ *                               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
+
+/*
+ ******************************************************************************
+ *               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_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 (32 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
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 28
+ * +----------------+----------------+----------------+----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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
+ * 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;
+};
+
+/*
+ * 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                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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.
+ */
+
+/*
+ *---------------------------------- Responses --------------------------------
+ *
+ * All response packets have the same length (32 octets)
+ *
+ * Response for all requests:
+ *         0                1                 2               3        octet
+ * +----------------+----------------+----------------+----------------+
+ * |               id                |    operation   |    reserved    | 4
+ * +----------------+----------------+----------------+----------------+
+ * |                              status                               | 8
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 12
+ * +----------------+----------------+----------------+----------------+
+ * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
+ * +----------------+----------------+----------------+----------------+
+ * |                             reserved                              | 32
+ * +----------------+----------------+----------------+----------------+
+ *
+ * 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
+ */
+
+struct xensnd_req {
+	uint16_t id;
+	uint8_t operation;
+	uint8_t reserved[5];
+	union {
+		struct xensnd_open_req open;
+		struct xensnd_rw_req rw;
+		uint8_t reserved[24];
+	} op;
+};
+
+struct xensnd_resp {
+	uint16_t id;
+	uint8_t operation;
+	uint8_t reserved;
+	int32_t status;
+	uint8_t reserved1[24];
+};
+
+DEFINE_RING_TYPES(xen_sndif, struct xensnd_req, struct xensnd_resp);
+
+#endif /* __XEN_PUBLIC_IO_SNDIF_H__ */