path: root/virtio-sound.tex

\section{Sound Device}\label{sec:Device Types / Sound Device}

The virtio sound card is a virtual audio device supporting input and output PCM
streams.

A device is managed by control requests and can send various notifications
through dedicated queues. A driver can transmit PCM frames using message-based
transport or shared memory.

A small part of the specification reuses existing layouts and values from the
High Definition Audio specification (\hyperref[intro:HDA]{HDA}). It allows to
provide the same functionality and assist in two possible cases:

\begin{enumerate}
\item implementation of a universal sound driver,
\item implementation of a sound driver as part of the High Definition Audio
subsystem.
\end{enumerate}

\subsection{Device ID}\label{sec:Device Types / Sound Device / Device ID}

25

\subsection{Virtqueues}\label{sec:Device Types / Sound Device / Virtqueues}

\begin{description}
\item[0] controlq
\item[1] eventq
\item[2] txq
\item[3] rxq
\end{description}

The control queue is used for sending control messages from the driver to
the device.

The event queue is used for sending notifications from the device to the driver.

The tx queue is used to send PCM frames for output streams.

The rx queue is used to receive PCM frames from input streams.

\subsection{Feature bits}\label{sec:Device Types / Sound Device / Feature bits}

None currently defined.

\subsection{Device configuration layout}\label{sec:Device Types / Sound Device / Device configuration layout}

\begin{lstlisting}
struct virtio_snd_config {
    le32 jacks;
    le32 streams;
};
\end{lstlisting}

A configuration space contains the following fields:

\begin{description}
\item[\field{jacks}] (driver-read-only) indicates a total number of all available
jacks.
\item[\field{streams}] (driver-read-only) indicates a total number of all available
PCM streams.
\end{description}

\subsection{Device Initialization}

\begin{enumerate}
\item Configure the control, event, tx and rx queues.
\item Read the \field{jacks} field and send control requests to query configuration
for each available jack.
\item Read the \field{streams} field and send control requests to query configuration
for each available PCM stream.
\item Populate the event queue with empty buffers.
\end{enumerate}

\devicenormative{\subsubsection}{Device Initialization}{Device Types / Sound Device / Device Initialization}

\begin{itemize}
\item The device MUST set a non-zero value for the \field{streams} configuration
field.
\end{itemize}

\drivernormative{\subsubsection}{Device Initialization}{Device Types / Sound Device / Device Initialization}

\begin{itemize}
\item The driver MUST populate the event queue with empty buffers of at least
the struct virtio_snd_event size.
\item The driver MUST NOT put a device-readable buffers in the event queue.
\end{itemize}

\subsection{Device Operation}\label{sec:Device Types / Sound Device / Device Operation}

All control messages are placed into the control queue and all notifications
are placed into the event queue. They use the following layout structure and
definitions:

\begin{lstlisting}
enum {
    /* jack control request types */
    VIRTIO_SND_R_JACK_GET_CONFIG = 1,
    VIRTIO_SND_R_JACK_REMAP,

    /* PCM control request types */
    VIRTIO_SND_R_PCM_GET_CONFIG = 0x0100,
    VIRTIO_SND_R_PCM_SET_PARAMS,
    VIRTIO_SND_R_PCM_PREPARE,
    VIRTIO_SND_R_PCM_RELEASE,
    VIRTIO_SND_R_PCM_START,
    VIRTIO_SND_R_PCM_STOP,

    /* jack event types */
    VIRTIO_SND_EVT_JACK_CONNECTED = 0x1000,
    VIRTIO_SND_EVT_JACK_DISCONNECTED,

    /* PCM event types */
    VIRTIO_SND_EVT_PCM_PERIOD_ELAPSED = 0x1100,
    VIRTIO_SND_EVT_PCM_XRUN,

    /* common status codes */
    VIRTIO_SND_S_OK = 0x8000,
    VIRTIO_SND_S_BAD_MSG,
    VIRTIO_SND_S_NOT_SUPP,
    VIRTIO_SND_S_IO_ERR
};

/* a common header */
struct virtio_snd_hdr {
    le32 code;
};

/* an event notification */
struct virtio_snd_event {
    struct virtio_snd_hdr hdr;
    le32 data;
};
\end{lstlisting}

A generic control message consists of a request part and a response part.

A request part has, or consists of, a common header containing the following
device-readable field:

\begin{description}
\item[\field{code}] specifies a device request type (VIRTIO_SND_R_*).
\end{description}

A response part has, or consists of, a common header containing the following
device-writable field:

\begin{description}
\item[\field{code}] indicates a device request status (VIRTIO_SND_S_*).
\end{description}

The status field can take one of the following values:

\begin{itemize}
\item VIRTIO_SND_S_OK - success.
\item VIRTIO_SND_S_BAD_MSG - a control message is malformed or contains invalid
parameters.
\item VIRTIO_SND_S_NOT_SUPP - requested operation or parameters are not supported.
\item VIRTIO_SND_S_IO_ERR - an I/O error occurred.
\end{itemize}

The request part may be followed by an additional device-readable payload,
and the response part may be followed by an additional device-writable payload.

An event notification contains the following device-writable fields:

\begin{description}
\item[\field{hdr}] indicates an event type (VIRTIO_SND_EVT_*).
\item[\field{data}] indicates an optional event data.
\end{description}

For all entities involved in the exchange of audio data, the device uses one of
the following data flow directions:

\begin{lstlisting}
enum {
    VIRTIO_SND_D_OUTPUT = 0,
    VIRTIO_SND_D_INPUT
};
\end{lstlisting}

\subsubsection{Relationships with the High Definition Audio specification}\label{sec:Device Types / Sound Device / Device Operation / Relationships with HDA}

The High Definition Audio specification introduces the codec as part of the
hardware that implements some of the functionality. The codec architecture and
capabilities are described by tree structure of special nodes each of which is
either a function module or a function group (see \hyperref[intro:HDA]{HDA} for
details).

The virtio sound specification assumes that a single codec is implemented in the
device. Function module nodes are simulated by entity configuration structures,
and functional group nodes are simulated by the \field{hda_fn_nid} field in each
such structure.

\subsubsection{Jack Control Messages}\label{sec:Device Types / Sound Device / Device Operation / Jack Control Messages}

A jack control request has, or consists of, a common header with the following
layout structure:

\begin{lstlisting}
struct virtio_snd_jack_hdr {
    struct virtio_snd_hdr hdr;
    le32 jack_id;
};
\end{lstlisting}

The header consists of the following device-readable fields:

\begin{description}
\item[\field{hdr}] specifies a request type (VIRTIO_SND_R_JACK_*).
\item[\field{jack_id}] specifies a jack identifier from 0 to \field{jacks} - 1.
\end{description}

\paragraph{VIRTIO_SND_R_JACK_GET_CONFIG}

Query a jack configuration for the specified jack ID.

The request has no additional payload, and the response uses the following
structure and layout definitions:

\begin{lstlisting}
/* supported jack features */
enum {
    VIRTIO_SND_JACK_F_REMAP = 0
};

/* a response containing jack configuration */
struct virtio_snd_jack_config {
    struct virtio_snd_hdr hdr; /* VIRTIO_SND_S_XXX */
    le32 features; /* 1 << VIRTIO_SND_JACK_F_XXX */
    le32 hda_fn_nid;
    le32 hda_reg_defconf;
    le32 hda_reg_caps;
    u8 connected;

    u8 padding[3];
};
\end{lstlisting}

The response contains the following device-writable fields:

\begin{description}
\item[\field{features}] indicates a supported feature bit map:
\begin{itemize}
\item VIRTIO_SND_JACK_F_REMAP - jack remapping support.
\end{itemize}
\item[\field{hda_fn_nid}] indicates a functional node identifier
(see \hyperref[intro:HDA]{HDA}, section 7.1.2).
\item[\field{hda_reg_defconf}] indicates a pin default configuration value
(see \hyperref[intro:HDA]{HDA}, section 7.3.3.31).
\item[\field{hda_reg_caps}] indicates a pin capabilities value
(see \hyperref[intro:HDA]{HDA}, section 7.3.4.9).
\item[\field{connected}] indicates the current jack connection status
(1 - connected, 0 - disconnected).
\end{description}

\devicenormative{\subparagraph}{Jack Configuration}{Device Types / Sound Device / Device Operation / Jack Configuration}

\begin{itemize}
\item The device MUST NOT set undefined feature values.
\item The device MUST initialize the \field{padding} bytes to 0.
\end{itemize}

\paragraph{VIRTIO_SND_R_JACK_REMAP}

If the VIRTIO_SND_JACK_F_REMAP feature bit is set in the jack configuration,
then the driver can send a control request to change the association and/or
sequence number for the specified jack ID.

The request uses the following structure and layout definitions:

\begin{lstlisting}
struct virtio_snd_jack_remap {
    struct virtio_snd_jack_hdr hdr; /* .code = VIRTIO_SND_R_JACK_REMAP */
    le32 association;
    le32 sequence;
};
\end{lstlisting}

The request contains the following device-readable fields:

\begin{description}
\item[\field{association}] specifies the selected association number.
\item[\field{sequence}] specifies the selected sequence number.
\end{description}

\subsubsection{Jack Notifications}

Jack notifications consist of a virtio_snd_event structure, which has the following
device-writable fields:

\begin{description}
\item[\field{hdr}] indicates a jack event type:
\begin{itemize}
\item VIRTIO_SND_EVT_JACK_CONNECTED - an external device has been connected to the jack.
\item VIRTIO_SND_EVT_JACK_DISCONNECTED - an external device has been disconnected from the jack.
\end{itemize}
\item[\field{data}] indicates a jack identifier from 0 to \field{jacks} - 1.
\end{description}

\subsubsection{PCM Control Messages}\label{sec:Device Types / Sound Device / Device Operation / PCM Control Messages}

A PCM control request has, or consists of, a common header with the following
layout structure:

\begin{lstlisting}
struct virtio_snd_pcm_hdr {
    struct virtio_snd_hdr hdr;
    le32 stream_id;
};
\end{lstlisting}

The header consists of the following device-readable fields:

\begin{description}
\item[\field{hdr}] specifies request type (VIRTIO_SND_R_PCM_*).
\item[\field{stream_id}] specifies a PCM stream identifier from 0 to \field{streams} - 1.
\end{description}

\paragraph{PCM Command Lifecycle}

A PCM stream has the following command lifecycle:

\begin{enumerate}
\item SET PARAMETERS

The driver negotiates the stream parameters (format, transport, etc) with
the device.

Possible valid transitions: set parameters, prepare.

\item PREPARE

The device prepares the stream (allocates resources, etc).

Possible valid transitions: set parameters, prepare, start, release.

\item Output only: the driver transfers data for pre-buffing.

\item START

The device starts the stream (unmute, putting into running state, etc).

Possible valid transitions: stop.

\item The driver transfers data to/from the stream.

\item STOP

The device stops the stream (mute, putting into non-running state, etc).

Possible valid transitions: start, release.

\item RELEASE

The device releases the stream (frees resources, etc).

Possible valid transitions: set parameters, prepare.

\end{enumerate}

\paragraph{VIRTIO_SND_R_PCM_GET_CONFIG}

Query stream configuration for the specified stream ID.

The request has no additional payload, and the response uses the following
structure and layout definitions:

\begin{lstlisting}
/* supported PCM stream features */
enum {
    VIRTIO_SND_PCM_F_SHMEM_HOST = 0,
    VIRTIO_SND_PCM_F_SHMEM_GUEST,
    VIRTIO_SND_PCM_F_MSG_POLLING,
    VIRTIO_SND_PCM_F_EVT_SHMEM_PERIODS,
    VIRTIO_SND_PCM_F_EVT_XRUNS
};

/* additional PCM stream flags */
enum {
    VIRTIO_SND_PCM_FL_CHMAP = 0
};

/* supported PCM sample formats */
enum {
    /* analog formats (width / physical width) */
    VIRTIO_SND_PCM_FMT_IMA_ADPCM = 0,   /*  4 /  4 bits */
    VIRTIO_SND_PCM_FMT_MU_LAW,          /*  8 /  8 bits */
    VIRTIO_SND_PCM_FMT_A_LAW,           /*  8 /  8 bits */
    VIRTIO_SND_PCM_FMT_S8,              /*  8 /  8 bits */
    VIRTIO_SND_PCM_FMT_U8,              /*  8 /  8 bits */
    VIRTIO_SND_PCM_FMT_S16,             /* 16 / 16 bits */
    VIRTIO_SND_PCM_FMT_U16,             /* 16 / 16 bits */
    VIRTIO_SND_PCM_FMT_S18_3,           /* 18 / 24 bits */
    VIRTIO_SND_PCM_FMT_U18_3,           /* 18 / 24 bits */
    VIRTIO_SND_PCM_FMT_S20_3,           /* 20 / 24 bits */
    VIRTIO_SND_PCM_FMT_U20_3,           /* 20 / 24 bits */
    VIRTIO_SND_PCM_FMT_S24_3,           /* 24 / 24 bits */
    VIRTIO_SND_PCM_FMT_U24_3,           /* 24 / 24 bits */
    VIRTIO_SND_PCM_FMT_S20,             /* 20 / 32 bits */
    VIRTIO_SND_PCM_FMT_U20,             /* 20 / 32 bits */
    VIRTIO_SND_PCM_FMT_S24,             /* 24 / 32 bits */
    VIRTIO_SND_PCM_FMT_U24,             /* 24 / 32 bits */
    VIRTIO_SND_PCM_FMT_S32,             /* 32 / 32 bits */
    VIRTIO_SND_PCM_FMT_U32,             /* 32 / 32 bits */
    VIRTIO_SND_PCM_FMT_FLOAT,           /* 32 / 32 bits */
    VIRTIO_SND_PCM_FMT_FLOAT64,         /* 64 / 64 bits */
    /* digital formats (width / physical width) */
    VIRTIO_SND_PCM_FMT_DSD_U8,          /*  8 /  8 bits */
    VIRTIO_SND_PCM_FMT_DSD_U16,         /* 16 / 16 bits */
    VIRTIO_SND_PCM_FMT_DSD_U32,         /* 32 / 32 bits */
    VIRTIO_SND_PCM_FMT_IEC958_SUBFRAME  /* 32 / 32 bits */
};

/* supported PCM frame rates */
enum {
    VIRTIO_SND_PCM_RATE_5512 = 0,
    VIRTIO_SND_PCM_RATE_8000,
    VIRTIO_SND_PCM_RATE_11025,
    VIRTIO_SND_PCM_RATE_16000,
    VIRTIO_SND_PCM_RATE_22050,
    VIRTIO_SND_PCM_RATE_32000,
    VIRTIO_SND_PCM_RATE_44100,
    VIRTIO_SND_PCM_RATE_48000,
    VIRTIO_SND_PCM_RATE_64000,
    VIRTIO_SND_PCM_RATE_88200,
    VIRTIO_SND_PCM_RATE_96000,
    VIRTIO_SND_PCM_RATE_176400,
    VIRTIO_SND_PCM_RATE_192000,
    VIRTIO_SND_PCM_RATE_384000
};

/* standard channel position definition */
enum {
    VIRTIO_SND_PCM_CH_NONE = 0,	/* undefined */
    VIRTIO_SND_PCM_CH_NA,       /* silent */
    VIRTIO_SND_PCM_CH_MONO,     /* mono stream */
    VIRTIO_SND_PCM_CH_FL,       /* front left */
    VIRTIO_SND_PCM_CH_FR,       /* front right */
    VIRTIO_SND_PCM_CH_RL,       /* rear left */
    VIRTIO_SND_PCM_CH_RR,       /* rear right */
    VIRTIO_SND_PCM_CH_FC,       /* front center */
    VIRTIO_SND_PCM_CH_LFE,      /* low frequency (LFE) */
    VIRTIO_SND_PCM_CH_SL,       /* side left */
    VIRTIO_SND_PCM_CH_SR,       /* side right */
    VIRTIO_SND_PCM_CH_RC,       /* rear center */
    VIRTIO_SND_PCM_CH_FLC,      /* front left center */
    VIRTIO_SND_PCM_CH_FRC,      /* front right center */
    VIRTIO_SND_PCM_CH_RLC,      /* rear left center */
    VIRTIO_SND_PCM_CH_RRC,      /* rear right center */
    VIRTIO_SND_PCM_CH_FLW,      /* front left wide */
    VIRTIO_SND_PCM_CH_FRW,      /* front right wide */
    VIRTIO_SND_PCM_CH_FLH,      /* front left high */
    VIRTIO_SND_PCM_CH_FCH,      /* front center high */
    VIRTIO_SND_PCM_CH_FRH,      /* front right high */
    VIRTIO_SND_PCM_CH_TC,       /* top center */
    VIRTIO_SND_PCM_CH_TFL,      /* top front left */
    VIRTIO_SND_PCM_CH_TFR,      /* top front right */
    VIRTIO_SND_PCM_CH_TFC,      /* top front center */
    VIRTIO_SND_PCM_CH_TRL,      /* top rear left */
    VIRTIO_SND_PCM_CH_TRR,      /* top rear right */
    VIRTIO_SND_PCM_CH_TRC,      /* top rear center */
    VIRTIO_SND_PCM_CH_TFLC,     /* top front left center */
    VIRTIO_SND_PCM_CH_TFRC,     /* top front right center */
    VIRTIO_SND_PCM_CH_TSL,      /* top side left */
    VIRTIO_SND_PCM_CH_TSR,      /* top side right */
    VIRTIO_SND_PCM_CH_LLFE,     /* left LFE */
    VIRTIO_SND_PCM_CH_RLFE,     /* right LFE */
    VIRTIO_SND_PCM_CH_BC,       /* bottom center */
    VIRTIO_SND_PCM_CH_BLC,      /* bottom left center */
    VIRTIO_SND_PCM_CH_BRC       /* bottom right center */
};

/* a maximum possible number of channels */
#define VIRTIO_SND_PCM_CH_MAX   16

/* a response containing PCM stream configuration */
struct virtio_snd_pcm_config {
    struct virtio_snd_hdr hdr; /* VIRTIO_SND_S_XXX */
    le32 hda_fn_nid;
    le32 features; /* 1 << VIRTIO_SND_PCM_F_XXX */
    u8 flags; /* 1 << VIRTIO_SND_PCM_FL_XXX */
    u8 direction;
    u8 channels_min;
    u8 channels_max;
    le64 formats; /* 1 << VIRTIO_SND_PCM_FMT_XXX */
    le64 rates; /* 1 << VIRTIO_SND_PCM_RATE_XXX */
    u8 chmap[VIRTIO_SND_PCM_CH_MAX];
};
\end{lstlisting}

The response contains the following device-writable fields:

\begin{description}
\item[\field{hda_fn_nid}] indicates a functional node identifier
(see \hyperref[intro:HDA]{HDA}, section 7.1.2).
\item[\field{features}] indicates a bit map of the supported features, which can
be negotiated by setting the stream parameters:
\begin{itemize}
\item VIRTIO_SND_PCM_F_SHMEM_HOST - supports sharing a host memory with a guest,
\item VIRTIO_SND_PCM_F_SHMEM_GUEST - supports sharing a guest memory with a host,
\item VIRTIO_SND_PCM_F_MSG_POLLING - supports polling mode for message-based
transport,
\item VIRTIO_SND_PCM_F_EVT_SHMEM_PERIODS - supports elapsed period notifications
for shared memory transport,
\item VIRTIO_SND_PCM_F_EVT_XRUNS - supports underrun/overrun notifications.
\end{itemize}
\item[\field{flags}] indicates a bit map of the additional flags:
\begin{itemize}
\item VIRTIO_SND_PCM_FL_CHMAP - supports a channel map.
\end{itemize}
\item[\field{direction}] indicates the direction of data flow (VIRTIO_SND_D_XXX).
\item[\field{channels_min}] indicates a minimum number of supported channels.
\item[\field{channels_max}] indicates a maximum number of supported channels.
\item[\field{formats}] indicates a supported sample format bit map.
\item[\field{rates}] indicates a supported frame rate bit map.
\item[\field{chmap}] if the VIRTIO_SND_PCM_FL_CHMAP flag is set, then the
first \field{channels_max} elements are valid and contain channel positions
(VIRTIO_SND_PCM_CH_*).
\end{description}

Only interleaved samples are supported.

\devicenormative{\subparagraph}{Stream Configuration}{Device Types / Sound Device / Device Operation / PCM Stream Configuration}

\begin{itemize}
\item The device MUST NOT set undefined direction, feature, format and rate
values.
\item If the VIRTIO_SND_PCM_FL_CHMAP flag is not set, then the device MUST
initialize the \field{chmap} field with the VIRTIO_SND_PCM_CH_NONE position
values.
\end{itemize}

\paragraph{VIRTIO_SND_R_PCM_SET_PARAMS}\label{sec:Device Types / Sound Device / Device Operation / PCM Stream Parameters}

Set selected stream parameters for the specified stream ID.

The request uses the following structure and layout definitions:

\begin{lstlisting}
struct virtio_snd_pcm_set_params {
    struct virtio_snd_pcm_hdr hdr; /* .code = VIRTIO_SND_R_PCM_SET_PARAMS */
    le32 buffer_bytes;
    le32 period_bytes;
    le32 features; /* 1 << VIRTIO_SND_PCM_F_XXX */
    u8 channels;
    u8 format;
    u8 rate;

    u8 padding;
};
\end{lstlisting}

The request contains the following device-readable fields:

\begin{description}
\item[\field{buffer_bytes}] specifies the size of the hardware buffer used by
the driver.
\item[\field{period_bytes}] specifies the size of the hardware period used by
the driver.
\item[\field{features}] specifies a selected feature bit map:
\begin{itemize}
\item VIRTIO_SND_PCM_F_SHMEM_HOST - use shared memory allocated by the host
(is a placeholder and MUST NOT be selected at the moment),
\item VIRTIO_SND_PCM_F_SHMEM_GUEST - use shared memory allocated by the guest
(is a placeholder and MUST NOT be selected at the moment),
\item VIRTIO_SND_PCM_F_MSG_POLLING - suppress available buffer notifications
for tx and rx queues (device should poll virtqueue),
\item VIRTIO_SND_PCM_F_EVT_SHMEM_PERIODS - enable elapsed period notifications
for shared memory transport,
\item VIRTIO_SND_PCM_F_EVT_XRUNS - enable underrun/overrun notifications.
\end{itemize}
\item[\field{channels}] specifies a selected number of channels.
\item[\field{format}] specifies a selected sample format (VIRTIO_SND_PCM_FMT_*).
\item[\field{rate}] specifies a selected frame rate (VIRTIO_SND_PCM_RATE_*).
\end{description}

\devicenormative{\subparagraph}{Stream Parameters}{Device Types / Sound Device / Device Operation / PCM Stream Parameters}

\begin{itemize}
\item If the device has an intermediate buffer, its size MUST be no less than
the specified \field{buffer_bytes} value.
\end{itemize}

\drivernormative{\subparagraph}{Stream Parameters}{Device Types / Sound Device / Device Operation / PCM Stream Parameters}

\begin{itemize}
\item \field{period_bytes} MUST be a divider \field{buffer_bytes}, i.e. \field{buffer_bytes} \% \field{period_bytes} == 0.
\item The driver MUST NOT set undefined feature, format and rate values.
\item The driver MUST NOT set the feature, format, and rate that are not specified
in the stream configuration.
\item The driver MUST NOT set the \field{channels} value as less than the \field{channels_min}
or greater than the \field{channels_max} values specified in the stream configuration.
\item The driver MUST NOT set the VIRTIO_SND_PCM_F_SHMEM_HOST and VIRTIO_SND_PCM_F_SHMEM_GUEST
bits at the same time.
\item The driver MUST initialize the \field{padding} byte to 0.
\item If the bits associated with the shared memory are not selected, the driver
MUST use the tx and rx queues for data transfer
(see \nameref{sec:Device Types / Sound Device / Device Operation / PCM IO Messages}).
\end{itemize}

\paragraph{VIRTIO_SND_R_PCM_PREPARE}

Prepare a stream with specified stream ID.

\paragraph{VIRTIO_SND_R_PCM_RELEASE}

Release a stream with specified stream ID.

\devicenormative{\subparagraph}{Stream Release}{Device Types / Sound Device / Device Operation / PCM Stream Release}

\begin{itemize}
\item The device MUST complete all pending I/O messages for the specified
stream ID.
\item The device MUST NOT complete the control request while there are pending
I/O messages for the specified stream ID.
\end{itemize}

\paragraph{VIRTIO_SND_R_PCM_START}

Start a stream with specified stream ID.

\paragraph{VIRTIO_SND_R_PCM_STOP}

Stop a stream with specified stream ID.

\subsubsection{PCM Notifications}

The device can announce support for different PCM events using feature bits
in the stream configuration structure. To enable notifications, the driver
must negotiate these features using the set stream parameters request
(see \ref{sec:Device Types / Sound Device / Device Operation / PCM Stream Parameters}).

PCM stream notifications consist of a virtio_snd_event structure, which has the
following device-writable fields:

\begin{description}
\item[\field{hdr}] indicates a PCM stream event type:
\begin{itemize}
\item VIRTIO_SND_EVT_PCM_PERIOD_ELAPSED - a hardware buffer period has elapsed,
the period size is controlled using the \field{period_bytes} field.
\item VIRTIO_SND_EVT_PCM_XRUN - an underflow for the output stream or an overflow
for the input stream has occurred.
\end{itemize}
\item[\field{data}] indicates a PCM stream identifier from 0 to \field{streams} - 1.
\end{description}

\subsubsection{PCM I/O Messages}\label{sec:Device Types / Sound Device / Device Operation / PCM IO Messages}

An I/O message consists of the header part, followed by the buffer, and then
the status part.

\begin{lstlisting}
/* an I/O header */
struct virtio_snd_pcm_xfer {
    le32 stream_id;
};

/* an I/O status */
struct virtio_snd_pcm_status {
    le32 status;
    le32 latency_bytes;
};
\end{lstlisting}

The header part consists of the following device-readable field:

\begin{description}
\item[\field{stream_id}] specifies a PCM stream identifier from 0 to \field{streams} - 1.
\end{description}

The status part consists of the following device-writable fields:

\begin{description}
\item[\field{status}] contains VIRTIO_SND_S_OK if an operation is successful,
and VIRTIO_SND_S_IO_ERR otherwise.
\item[\field{latency_bytes}] indicates the current device latency.
\end{description}

Since all buffers in the queue (with one exception) should be of the size
\field{period_bytes}, the completion of such an I/O request can be considered an
elapsed period notification.

\paragraph{Output Stream}

In case of an output stream, the header is followed by a device-readable buffer
containing PCM frames for writing to the device. All messages are placed into
the tx queue.

\devicenormative{\subparagraph}{Output Stream}{Device Types / Sound Device / Device Operation / PCM Output Stream}

\begin{itemize}
\item The device MUST NOT complete the I/O request until the buffer is totally
consumed.
\end{itemize}

\drivernormative{\subparagraph}{Output Stream}{Device Types / Sound Device / Device Operation / PCM Output Stream}

\begin{itemize}
\item The driver SHOULD populate the tx queue with \field{period_bytes} sized
buffers. The only exception is the end of the stream.
\item The driver MUST NOT place device-writable buffers into the tx queue.
\end{itemize}

\paragraph{Input Stream}

In case of an input stream, the header is followed by a device-writable buffer
being populated with PCM frames from the device. All messages are placed into
the rx queue.

A used descriptor specifies the length of the buffer that was written by the
device. It should be noted that the length value contains the size of the
virtio_snd_pcm_status structure plus the size of the recorded frames.

\devicenormative{\subparagraph}{Input Stream}{Device Types / Sound Device / Device Operation / PCM Input Stream}

\begin{itemize}
\item The device MUST NOT complete the I/O request until the buffer is full.
The only exception is the end of the stream.
\end{itemize}

\drivernormative{\subparagraph}{Input Stream}{Device Types / Sound Device / Device Operation / PCM Input Stream}

\begin{itemize}
\item The driver SHOULD populate the rx queue with \field{period_bytes} sized
empty buffers before starting the stream.
\item The driver MUST NOT place device-readable buffers into the rx queue.
\end{itemize}