devices/src/virtio/snd/common_backend/async_funcs.rs

// Copyright 2021 The ChromiumOS Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use std::fmt;
use std::io;
use std::io::Read;
use std::io::Write;
use std::rc::Rc;

use async_trait::async_trait;
use audio_streams::capture::AsyncCaptureBuffer;
use audio_streams::AsyncPlaybackBuffer;
use base::debug;
use base::error;
use cros_async::sync::Condvar;
use cros_async::sync::Mutex as AsyncMutex;
use cros_async::EventAsync;
use cros_async::Executor;
use futures::channel::mpsc;
use futures::channel::oneshot;
use futures::pin_mut;
use futures::select;
use futures::FutureExt;
use futures::SinkExt;
use futures::StreamExt;
use thiserror::Error as ThisError;
use vm_memory::GuestMemory;
#[cfg(windows)]
use win_audio::AudioSharedFormat;
use zerocopy::AsBytes;

use super::Error;
use super::SndData;
use super::WorkerStatus;
use crate::virtio::snd::common::*;
use crate::virtio::snd::common_backend::stream_info::SetParams;
use crate::virtio::snd::common_backend::stream_info::StreamInfo;
use crate::virtio::snd::common_backend::DirectionalStream;
use crate::virtio::snd::common_backend::PcmResponse;
use crate::virtio::snd::constants::*;
use crate::virtio::snd::layout::*;
use crate::virtio::DescriptorChain;
use crate::virtio::Queue;
use crate::virtio::Reader;
use crate::virtio::SignalableInterrupt;
use crate::virtio::Writer;

// TODO(b/246601226): Remove once a generic audio_stream solution that can accpet
// arbitrarily size buffers.
/// Trait to wrap system specific helpers for writing to endpoint playback buffers.
#[async_trait(?Send)]
pub trait PlaybackBufferWriter {
    fn new(
        guest_period_bytes: usize,
        #[cfg(windows)] frame_size: usize,
        #[cfg(windows)] frame_rate: usize,
        #[cfg(windows)] guest_num_channels: usize,
        #[cfg(windows)] audio_shared_format: AudioSharedFormat,
    ) -> Self
    where
        Self: Sized;

    /// Returns the period of the endpoint device.
    fn endpoint_period_bytes(&self) -> usize;

    /// Read audio samples from the tx virtqueue.
    fn copy_to_buffer(
        &mut self,
        dst_buf: &mut AsyncPlaybackBuffer<'_>,
        reader: &mut Reader,
    ) -> Result<usize, Error> {
        dst_buf.copy_from(reader).map_err(Error::Io)
    }
    /// Check to see if an additional read from the tx virtqueue is needed during a playback
    /// loop. If so, read from the virtqueue.
    ///
    /// Prefill will happen, for example, if the endpoint buffer requires a 513 frame period, but
    /// each tx virtqueue read only produces 480 frames.
    #[cfg(windows)]
    async fn check_and_prefill(
        &mut self,
        desc_receiver: &mut mpsc::UnboundedReceiver<DescriptorChain>,
        sender: &mut mpsc::UnboundedSender<PcmResponse>,
    ) -> Result<(), Error>;
}

#[derive(Debug)]
enum VirtioSndPcmCmd {
    SetParams { set_params: SetParams },
    Prepare,
    Start,
    Stop,
    Release,
}

impl fmt::Display for VirtioSndPcmCmd {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let cmd_code = match self {
            VirtioSndPcmCmd::SetParams { set_params: _ } => VIRTIO_SND_R_PCM_SET_PARAMS,
            VirtioSndPcmCmd::Prepare => VIRTIO_SND_R_PCM_PREPARE,
            VirtioSndPcmCmd::Start => VIRTIO_SND_R_PCM_START,
            VirtioSndPcmCmd::Stop => VIRTIO_SND_R_PCM_STOP,
            VirtioSndPcmCmd::Release => VIRTIO_SND_R_PCM_RELEASE,
        };
        f.write_str(get_virtio_snd_r_pcm_cmd_name(cmd_code))
    }
}

#[derive(ThisError, Debug)]
enum VirtioSndPcmCmdError {
    #[error("SetParams requires additional parameters")]
    SetParams,
    #[error("Invalid virtio snd command code")]
    InvalidCode,
}

impl TryFrom<u32> for VirtioSndPcmCmd {
    type Error = VirtioSndPcmCmdError;

    fn try_from(code: u32) -> Result<Self, Self::Error> {
        match code {
            VIRTIO_SND_R_PCM_PREPARE => Ok(VirtioSndPcmCmd::Prepare),
            VIRTIO_SND_R_PCM_START => Ok(VirtioSndPcmCmd::Start),
            VIRTIO_SND_R_PCM_STOP => Ok(VirtioSndPcmCmd::Stop),
            VIRTIO_SND_R_PCM_RELEASE => Ok(VirtioSndPcmCmd::Release),
            VIRTIO_SND_R_PCM_SET_PARAMS => Err(VirtioSndPcmCmdError::SetParams),
            _ => Err(VirtioSndPcmCmdError::InvalidCode),
        }
    }
}

impl VirtioSndPcmCmd {
    fn with_set_params_and_direction(
        set_params: virtio_snd_pcm_set_params,
        dir: u8,
    ) -> VirtioSndPcmCmd {
        let buffer_bytes: u32 = set_params.buffer_bytes.into();
        let period_bytes: u32 = set_params.period_bytes.into();
        VirtioSndPcmCmd::SetParams {
            set_params: SetParams {
                channels: set_params.channels,
                format: from_virtio_sample_format(set_params.format).unwrap(),
                frame_rate: from_virtio_frame_rate(set_params.rate).unwrap(),
                buffer_bytes: buffer_bytes as usize,
                period_bytes: period_bytes as usize,
                dir,
            },
        }
    }
}

// Returns true if the operation is successful. Returns error if there is
// a runtime/internal error
async fn process_pcm_ctrl(
    ex: &Executor,
    tx_send: &mpsc::UnboundedSender<PcmResponse>,
    rx_send: &mpsc::UnboundedSender<PcmResponse>,
    streams: &Rc<AsyncMutex<Vec<AsyncMutex<StreamInfo>>>>,
    cmd: VirtioSndPcmCmd,
    writer: &mut Writer,
    stream_id: usize,
) -> Result<(), Error> {
    let streams = streams.read_lock().await;
    let mut stream = match streams.get(stream_id) {
        Some(stream_info) => stream_info.lock().await,
        None => {
            error!(
                "Stream id={} not found for {}. Error code: VIRTIO_SND_S_BAD_MSG",
                stream_id, cmd
            );
            return writer
                .write_obj(VIRTIO_SND_S_BAD_MSG)
                .map_err(Error::WriteResponse);
        }
    };

    debug!("{} for stream id={}", cmd, stream_id);

    let result = match cmd {
        VirtioSndPcmCmd::SetParams { set_params } => {
            let result = stream.set_params(set_params).await;
            if result.is_ok() {
                debug!(
                    "VIRTIO_SND_R_PCM_SET_PARAMS for stream id={}. Stream info: {:#?}",
                    stream_id, *stream
                );
            }
            result
        }
        VirtioSndPcmCmd::Prepare => stream.prepare(ex, tx_send, rx_send).await,
        VirtioSndPcmCmd::Start => stream.start().await,
        VirtioSndPcmCmd::Stop => stream.stop().await,
        VirtioSndPcmCmd::Release => stream.release().await,
    };
    match result {
        Ok(_) => writer
            .write_obj(VIRTIO_SND_S_OK)
            .map_err(Error::WriteResponse),
        Err(Error::OperationNotSupported) => {
            error!(
                "{} for stream id={} failed. Error code: VIRTIO_SND_S_NOT_SUPP.",
                cmd, stream_id
            );

            writer
                .write_obj(VIRTIO_SND_S_NOT_SUPP)
                .map_err(Error::WriteResponse)
        }
        Err(e) => {
            // Runtime/internal error would be more appropriate, but there's
            // no such error type
            error!(
                "{} for stream id={} failed. Error code: VIRTIO_SND_S_IO_ERR. Actual error: {}",
                cmd, stream_id, e
            );
            writer
                .write_obj(VIRTIO_SND_S_IO_ERR)
                .map_err(Error::WriteResponse)
        }
    }
}

async fn write_data(
    mut dst_buf: AsyncPlaybackBuffer<'_>,
    reader: Option<&mut Reader>,
    buffer_writer: &mut Box<dyn PlaybackBufferWriter>,
) -> Result<u32, Error> {
    let transferred = match reader {
        Some(reader) => buffer_writer.copy_to_buffer(&mut dst_buf, reader)?,
        None => dst_buf
            .copy_from(&mut io::repeat(0).take(buffer_writer.endpoint_period_bytes() as u64))
            .map_err(Error::Io)?,
    };

    if transferred != buffer_writer.endpoint_period_bytes() {
        error!(
            "Bytes written {} != period_bytes {}",
            transferred,
            buffer_writer.endpoint_period_bytes()
        );
        Err(Error::InvalidBufferSize)
    } else {
        dst_buf.commit().await;
        Ok(dst_buf.latency_bytes())
    }
}

async fn read_data<'a>(
    mut src_buf: AsyncCaptureBuffer<'a>,
    writer: Option<&mut Writer>,
    period_bytes: usize,
) -> Result<u32, Error> {
    let transferred = match writer {
        Some(writer) => src_buf.copy_to(writer),
        None => src_buf.copy_to(&mut io::sink()),
    }
    .map_err(Error::Io)?;
    if transferred != period_bytes {
        error!(
            "Bytes written {} != period_bytes {}",
            transferred, period_bytes
        );
        Err(Error::InvalidBufferSize)
    } else {
        src_buf.commit().await;
        Ok(src_buf.latency_bytes())
    }
}

impl From<Result<u32, Error>> for virtio_snd_pcm_status {
    fn from(res: Result<u32, Error>) -> Self {
        match res {
            Ok(latency_bytes) => virtio_snd_pcm_status::new(StatusCode::OK, latency_bytes),
            Err(e) => {
                error!("PCM I/O message failed: {}", e);
                virtio_snd_pcm_status::new(StatusCode::IoErr, 0)
            }
        }
    }
}

// Drain all DescriptorChain in desc_receiver during WorkerStatus::Quit process.
async fn drain_desc_receiver(
    desc_receiver: &mut mpsc::UnboundedReceiver<DescriptorChain>,
    sender: &mut mpsc::UnboundedSender<PcmResponse>,
) -> Result<(), Error> {
    let mut o_desc_chain = desc_receiver.next().await;
    while let Some(desc_chain) = o_desc_chain {
        // From the virtio-snd spec:
        // The device MUST complete all pending I/O messages for the specified stream ID.
        let status = virtio_snd_pcm_status::new(StatusCode::OK, 0);
        // Fetch next DescriptorChain to see if the current one is the last one.
        o_desc_chain = desc_receiver.next().await;
        let (done, future) = if o_desc_chain.is_none() {
            let (done, future) = oneshot::channel();
            (Some(done), Some(future))
        } else {
            (None, None)
        };
        sender
            .send(PcmResponse {
                desc_chain,
                status,
                done,
            })
            .await
            .map_err(Error::MpscSend)?;

        if let Some(f) = future {
            // From the virtio-snd spec:
            // The device MUST NOT complete the control request (VIRTIO_SND_R_PCM_RELEASE)
            // while there are pending I/O messages for the specified stream ID.
            f.await.map_err(Error::DoneNotTriggered)?;
        };
    }
    Ok(())
}

/// Start a pcm worker that receives descriptors containing PCM frames (audio data) from the tx/rx
/// queue, and forward them to CRAS. One pcm worker per stream.
///
/// This worker is started when VIRTIO_SND_R_PCM_PREPARE is called, and returned before
/// VIRTIO_SND_R_PCM_RELEASE is completed for the stream.
pub async fn start_pcm_worker(
    ex: Executor,
    dstream: DirectionalStream,
    mut desc_receiver: mpsc::UnboundedReceiver<DescriptorChain>,
    status_mutex: Rc<AsyncMutex<WorkerStatus>>,
    mut sender: mpsc::UnboundedSender<PcmResponse>,
) -> Result<(), Error> {
    let res = pcm_worker_loop(ex, dstream, &mut desc_receiver, &status_mutex, &mut sender).await;
    *status_mutex.lock().await = WorkerStatus::Quit;
    if res.is_err() {
        error!(
            "pcm_worker error: {:#?}. Draining desc_receiver",
            res.as_ref().err()
        );
        // On error, guaranteed that desc_receiver has not been drained, so drain it here.
        // Note that drain blocks until the stream is release.
        drain_desc_receiver(&mut desc_receiver, &mut sender).await?;
    }
    res
}

async fn pcm_worker_loop(
    ex: Executor,
    dstream: DirectionalStream,
    desc_receiver: &mut mpsc::UnboundedReceiver<DescriptorChain>,
    status_mutex: &Rc<AsyncMutex<WorkerStatus>>,
    sender: &mut mpsc::UnboundedSender<PcmResponse>,
) -> Result<(), Error> {
    match dstream {
        #[allow(unused_mut)]
        DirectionalStream::Output(mut stream, mut buffer_writer) => loop {
            let dst_buf = stream
                .next_playback_buffer(&ex)
                .await
                .map_err(Error::FetchBuffer)?;
            let worker_status = status_mutex.lock().await;
            match *worker_status {
                WorkerStatus::Quit => {
                    drain_desc_receiver(desc_receiver, sender).await?;
                    if let Err(e) = write_data(dst_buf, None, &mut buffer_writer).await {
                        error!("Error on write_data after worker quit: {}", e)
                    }
                    break Ok(());
                }
                WorkerStatus::Pause => {
                    write_data(dst_buf, None, &mut buffer_writer).await?;
                }
                WorkerStatus::Running => {
                    // TODO(b/246601226): Remove once a generic audio_stream solution that can
                    // accpet arbitrarily size buffers
                    #[cfg(windows)]
                    buffer_writer
                        .check_and_prefill(desc_receiver, sender)
                        .await?;

                    match desc_receiver.try_next() {
                        Err(e) => {
                            error!("Underrun. No new DescriptorChain while running: {}", e);
                            write_data(dst_buf, None, &mut buffer_writer).await?;
                        }
                        Ok(None) => {
                            error!("Unreachable. status should be Quit when the channel is closed");
                            write_data(dst_buf, None, &mut buffer_writer).await?;
                            return Err(Error::InvalidPCMWorkerState);
                        }
                        Ok(Some(mut desc_chain)) => {
                            // stream_id was already read in handle_pcm_queue
                            let status = write_data(
                                dst_buf,
                                Some(&mut desc_chain.reader),
                                &mut buffer_writer,
                            )
                            .await
                            .into();
                            sender
                                .send(PcmResponse {
                                    desc_chain,
                                    status,
                                    done: None,
                                })
                                .await
                                .map_err(Error::MpscSend)?;
                        }
                    }
                }
            }
        },
        DirectionalStream::Input(mut stream, period_bytes) => loop {
            let src_buf = stream
                .next_capture_buffer(&ex)
                .await
                .map_err(Error::FetchBuffer)?;

            let worker_status = status_mutex.lock().await;
            match *worker_status {
                WorkerStatus::Quit => {
                    drain_desc_receiver(desc_receiver, sender).await?;
                    if let Err(e) = read_data(src_buf, None, period_bytes).await {
                        error!("Error on read_data after worker quit: {}", e)
                    }
                    break Ok(());
                }
                WorkerStatus::Pause => {
                    read_data(src_buf, None, period_bytes).await?;
                }
                WorkerStatus::Running => match desc_receiver.try_next() {
                    Err(e) => {
                        error!("Overrun. No new DescriptorChain while running: {}", e);
                        read_data(src_buf, None, period_bytes).await?;
                    }
                    Ok(None) => {
                        error!("Unreachable. status should be Quit when the channel is closed");
                        read_data(src_buf, None, period_bytes).await?;
                        return Err(Error::InvalidPCMWorkerState);
                    }
                    Ok(Some(mut desc_chain)) => {
                        let status = read_data(src_buf, Some(&mut desc_chain.writer), period_bytes)
                            .await
                            .into();
                        sender
                            .send(PcmResponse {
                                desc_chain,
                                status,
                                done: None,
                            })
                            .await
                            .map_err(Error::MpscSend)?;
                    }
                },
            }
        },
    }
}

// Defer pcm message response to the pcm response worker
async fn defer_pcm_response_to_worker(
    desc_chain: DescriptorChain,
    status: virtio_snd_pcm_status,
    response_sender: &mut mpsc::UnboundedSender<PcmResponse>,
) -> Result<(), Error> {
    response_sender
        .send(PcmResponse {
            desc_chain,
            status,
            done: None,
        })
        .await
        .map_err(Error::MpscSend)
}

fn send_pcm_response<I: SignalableInterrupt>(
    mut desc_chain: DescriptorChain,
    mem: &GuestMemory,
    queue: &mut Queue,
    interrupt: &I,
    status: virtio_snd_pcm_status,
) -> Result<(), Error> {
    let writer = &mut desc_chain.writer;

    // For rx queue only. Fast forward the unused audio data buffer.
    if writer.available_bytes() > std::mem::size_of::<virtio_snd_pcm_status>() {
        writer
            .consume_bytes(writer.available_bytes() - std::mem::size_of::<virtio_snd_pcm_status>());
    }
    writer.write_obj(status).map_err(Error::WriteResponse)?;
    let len = writer.bytes_written() as u32;
    queue.add_used(mem, desc_chain, len);
    queue.trigger_interrupt(mem, interrupt);
    Ok(())
}

// Await until reset_signal has been released
async fn await_reset_signal(reset_signal_option: Option<&(AsyncMutex<bool>, Condvar)>) {
    match reset_signal_option {
        Some((lock, cvar)) => {
            let mut reset = lock.lock().await;
            while !*reset {
                reset = cvar.wait(reset).await;
            }
        }
        None => futures::future::pending().await,
    };
}

pub async fn send_pcm_response_worker<I: SignalableInterrupt>(
    mem: &GuestMemory,
    queue: Rc<AsyncMutex<Queue>>,
    interrupt: I,
    recv: &mut mpsc::UnboundedReceiver<PcmResponse>,
    reset_signal: Option<&(AsyncMutex<bool>, Condvar)>,
) -> Result<(), Error> {
    let on_reset = await_reset_signal(reset_signal).fuse();
    pin_mut!(on_reset);

    loop {
        let next_async = recv.next().fuse();
        pin_mut!(next_async);

        let res = select! {
            _ = on_reset => break,
            res = next_async => res,
        };

        if let Some(r) = res {
            send_pcm_response(
                r.desc_chain,
                mem,
                &mut *queue.lock().await,
                &interrupt,
                r.status,
            )?;

            // Resume pcm_worker
            if let Some(done) = r.done {
                done.send(()).map_err(Error::OneshotSend)?;
            }
        } else {
            debug!("PcmResponse channel is closed.");
            break;
        }
    }
    Ok(())
}

/// Handle messages from the tx or the rx queue. One invocation is needed for
/// each queue.
pub async fn handle_pcm_queue(
    mem: &GuestMemory,
    streams: &Rc<AsyncMutex<Vec<AsyncMutex<StreamInfo>>>>,
    mut response_sender: mpsc::UnboundedSender<PcmResponse>,
    queue: Rc<AsyncMutex<Queue>>,
    queue_event: &EventAsync,
    reset_signal: Option<&(AsyncMutex<bool>, Condvar)>,
) -> Result<(), Error> {
    let on_reset = await_reset_signal(reset_signal).fuse();
    pin_mut!(on_reset);

    loop {
        // Manual queue.next_async() to avoid holding the mutex
        let next_async = async {
            loop {
                // Check if there are more descriptors available.
                if let Some(chain) = queue.lock().await.pop(mem) {
                    return Ok(chain);
                }
                queue_event.next_val().await?;
            }
        }
        .fuse();
        pin_mut!(next_async);

        let mut desc_chain = select! {
            _ = on_reset => break,
            res = next_async => res.map_err(Error::Async)?,
        };

        let pcm_xfer: virtio_snd_pcm_xfer =
            desc_chain.reader.read_obj().map_err(Error::ReadMessage)?;
        let stream_id: usize = u32::from(pcm_xfer.stream_id) as usize;

        let streams = streams.read_lock().await;
        let stream_info = match streams.get(stream_id) {
            Some(stream_info) => stream_info.read_lock().await,
            None => {
                error!(
                    "stream_id ({}) >= num_streams ({})",
                    stream_id,
                    streams.len()
                );
                defer_pcm_response_to_worker(
                    desc_chain,
                    virtio_snd_pcm_status::new(StatusCode::IoErr, 0),
                    &mut response_sender,
                )
                .await?;
                continue;
            }
        };

        match stream_info.sender.as_ref() {
            Some(mut s) => {
                s.send(desc_chain).await.map_err(Error::MpscSend)?;
                if *stream_info.status_mutex.lock().await == WorkerStatus::Quit {
                    // If sender channel is still intact but worker status is quit,
                    // the worker quitted unexpectedly. Return error to request a reset.
                    return Err(Error::PCMWorkerQuittedUnexpectedly);
                }
            }
            None => {
                if !stream_info.just_reset {
                    error!(
                        "stream {} is not ready. state: {}",
                        stream_id,
                        get_virtio_snd_r_pcm_cmd_name(stream_info.state)
                    );
                }
                defer_pcm_response_to_worker(
                    desc_chain,
                    virtio_snd_pcm_status::new(StatusCode::IoErr, 0),
                    &mut response_sender,
                )
                .await?;
            }
        };
    }
    Ok(())
}

/// Handle all the control messages from the ctrl queue.
pub async fn handle_ctrl_queue<I: SignalableInterrupt>(
    ex: &Executor,
    mem: &GuestMemory,
    streams: &Rc<AsyncMutex<Vec<AsyncMutex<StreamInfo>>>>,
    snd_data: &SndData,
    queue: Rc<AsyncMutex<Queue>>,
    queue_event: &mut EventAsync,
    interrupt: I,
    tx_send: mpsc::UnboundedSender<PcmResponse>,
    rx_send: mpsc::UnboundedSender<PcmResponse>,
    reset_signal: Option<&(AsyncMutex<bool>, Condvar)>,
) -> Result<(), Error> {
    let on_reset = await_reset_signal(reset_signal).fuse();
    pin_mut!(on_reset);

    let mut queue = queue.lock().await;
    loop {
        let mut desc_chain = {
            let next_async = queue.next_async(mem, queue_event).fuse();
            pin_mut!(next_async);

            select! {
                _ = on_reset => break,
                res = next_async => res.map_err(Error::Async)?,
            }
        };

        let reader = &mut desc_chain.reader;
        let writer = &mut desc_chain.writer;
        // Don't advance the reader
        let code = reader
            .clone()
            .read_obj::<virtio_snd_hdr>()
            .map_err(Error::ReadMessage)?
            .code
            .into();

        let handle_ctrl_msg = async {
            return match code {
                VIRTIO_SND_R_JACK_INFO => {
                    let query_info: virtio_snd_query_info =
                        reader.read_obj().map_err(Error::ReadMessage)?;
                    let start_id: usize = u32::from(query_info.start_id) as usize;
                    let count: usize = u32::from(query_info.count) as usize;
                    if start_id + count > snd_data.jack_info.len() {
                        error!(
                            "start_id({}) + count({}) must be smaller than \
                            the number of jacks ({})",
                            start_id,
                            count,
                            snd_data.jack_info.len()
                        );
                        return writer
                            .write_obj(VIRTIO_SND_S_BAD_MSG)
                            .map_err(Error::WriteResponse);
                    }
                    // The response consists of the virtio_snd_hdr structure (contains the request
                    // status code), followed by the device-writable information structures of the
                    // item. Each information structure begins with the following common header
                    writer
                        .write_obj(VIRTIO_SND_S_OK)
                        .map_err(Error::WriteResponse)?;
                    for i in start_id..(start_id + count) {
                        writer
                            .write_all(snd_data.jack_info[i].as_bytes())
                            .map_err(Error::WriteResponse)?;
                    }
                    Ok(())
                }
                VIRTIO_SND_R_PCM_INFO => {
                    let query_info: virtio_snd_query_info =
                        reader.read_obj().map_err(Error::ReadMessage)?;
                    let start_id: usize = u32::from(query_info.start_id) as usize;
                    let count: usize = u32::from(query_info.count) as usize;
                    if start_id + count > snd_data.pcm_info.len() {
                        error!(
                            "start_id({}) + count({}) must be smaller than \
                            the number of streams ({})",
                            start_id,
                            count,
                            snd_data.pcm_info.len()
                        );
                        return writer
                            .write_obj(VIRTIO_SND_S_BAD_MSG)
                            .map_err(Error::WriteResponse);
                    }
                    // The response consists of the virtio_snd_hdr structure (contains the request
                    // status code), followed by the device-writable information structures of the
                    // item. Each information structure begins with the following common header
                    writer
                        .write_obj(VIRTIO_SND_S_OK)
                        .map_err(Error::WriteResponse)?;
                    for i in start_id..(start_id + count) {
                        writer
                            .write_all(snd_data.pcm_info[i].as_bytes())
                            .map_err(Error::WriteResponse)?;
                    }
                    Ok(())
                }
                VIRTIO_SND_R_CHMAP_INFO => {
                    let query_info: virtio_snd_query_info =
                        reader.read_obj().map_err(Error::ReadMessage)?;
                    let start_id: usize = u32::from(query_info.start_id) as usize;
                    let count: usize = u32::from(query_info.count) as usize;
                    if start_id + count > snd_data.chmap_info.len() {
                        error!(
                            "start_id({}) + count({}) must be smaller than \
                            the number of chmaps ({})",
                            start_id,
                            count,
                            snd_data.chmap_info.len()
                        );
                        return writer
                            .write_obj(VIRTIO_SND_S_BAD_MSG)
                            .map_err(Error::WriteResponse);
                    }
                    // The response consists of the virtio_snd_hdr structure (contains the request
                    // status code), followed by the device-writable information structures of the
                    // item. Each information structure begins with the following common header
                    writer
                        .write_obj(VIRTIO_SND_S_OK)
                        .map_err(Error::WriteResponse)?;
                    for i in start_id..(start_id + count) {
                        writer
                            .write_all(snd_data.chmap_info[i].as_bytes())
                            .map_err(Error::WriteResponse)?;
                    }
                    Ok(())
                }
                VIRTIO_SND_R_JACK_REMAP => {
                    unreachable!("remap is unsupported");
                }
                VIRTIO_SND_R_PCM_SET_PARAMS => {
                    // Raise VIRTIO_SND_S_BAD_MSG or IO error?
                    let set_params: virtio_snd_pcm_set_params =
                        reader.read_obj().map_err(Error::ReadMessage)?;
                    let stream_id: usize = u32::from(set_params.hdr.stream_id) as usize;
                    let buffer_bytes: u32 = set_params.buffer_bytes.into();
                    let period_bytes: u32 = set_params.period_bytes.into();

                    let dir = match snd_data.pcm_info.get(stream_id) {
                        Some(pcm_info) => {
                            if set_params.channels < pcm_info.channels_min
                                || set_params.channels > pcm_info.channels_max
                            {
                                error!(
                                    "Number of channels ({}) must be between {} and {}",
                                    set_params.channels,
                                    pcm_info.channels_min,
                                    pcm_info.channels_max
                                );
                                return writer
                                    .write_obj(VIRTIO_SND_S_NOT_SUPP)
                                    .map_err(Error::WriteResponse);
                            }
                            if (u64::from(pcm_info.formats) & (1 << set_params.format)) == 0 {
                                error!("PCM format {} is not supported.", set_params.format);
                                return writer
                                    .write_obj(VIRTIO_SND_S_NOT_SUPP)
                                    .map_err(Error::WriteResponse);
                            }
                            if (u64::from(pcm_info.rates) & (1 << set_params.rate)) == 0 {
                                error!("PCM frame rate {} is not supported.", set_params.rate);
                                return writer
                                    .write_obj(VIRTIO_SND_S_NOT_SUPP)
                                    .map_err(Error::WriteResponse);
                            }

                            pcm_info.direction
                        }
                        None => {
                            error!(
                                "stream_id {} < streams {}",
                                stream_id,
                                snd_data.pcm_info.len()
                            );
                            return writer
                                .write_obj(VIRTIO_SND_S_BAD_MSG)
                                .map_err(Error::WriteResponse);
                        }
                    };

                    if set_params.features != 0 {
                        error!("No feature is supported");
                        return writer
                            .write_obj(VIRTIO_SND_S_NOT_SUPP)
                            .map_err(Error::WriteResponse);
                    }

                    if buffer_bytes % period_bytes != 0 {
                        error!(
                            "buffer_bytes({}) must be dividable by period_bytes({})",
                            buffer_bytes, period_bytes
                        );
                        return writer
                            .write_obj(VIRTIO_SND_S_BAD_MSG)
                            .map_err(Error::WriteResponse);
                    }

                    process_pcm_ctrl(
                        ex,
                        &tx_send,
                        &rx_send,
                        streams,
                        VirtioSndPcmCmd::with_set_params_and_direction(set_params, dir),
                        writer,
                        stream_id,
                    )
                    .await
                }
                VIRTIO_SND_R_PCM_PREPARE
                | VIRTIO_SND_R_PCM_START
                | VIRTIO_SND_R_PCM_STOP
                | VIRTIO_SND_R_PCM_RELEASE => {
                    let hdr: virtio_snd_pcm_hdr = reader.read_obj().map_err(Error::ReadMessage)?;
                    let stream_id: usize = u32::from(hdr.stream_id) as usize;
                    let cmd = match VirtioSndPcmCmd::try_from(code) {
                        Ok(cmd) => cmd,
                        Err(err) => {
                            error!("Error converting code to command: {}", err);
                            return writer
                                .write_obj(VIRTIO_SND_S_BAD_MSG)
                                .map_err(Error::WriteResponse);
                        }
                    };
                    process_pcm_ctrl(ex, &tx_send, &rx_send, streams, cmd, writer, stream_id)
                        .await
                        .and(Ok(()))?;
                    Ok(())
                }
                c => {
                    error!("Unrecognized code: {}", c);
                    return writer
                        .write_obj(VIRTIO_SND_S_BAD_MSG)
                        .map_err(Error::WriteResponse);
                }
            };
        };

        handle_ctrl_msg.await?;
        let len = writer.bytes_written() as u32;
        queue.add_used(mem, desc_chain, len);
        queue.trigger_interrupt(mem, &interrupt);
    }
    Ok(())
}

/// Send events to the audio driver.
pub async fn handle_event_queue<I: SignalableInterrupt>(
    mem: &GuestMemory,
    mut queue: Queue,
    mut queue_event: EventAsync,
    interrupt: I,
) -> Result<(), Error> {
    loop {
        let desc_chain = queue
            .next_async(mem, &mut queue_event)
            .await
            .map_err(Error::Async)?;

        // TODO(woodychow): Poll and forward events from cras asynchronously (API to be added)
        queue.add_used(mem, desc_chain, 0);
        queue.trigger_interrupt(mem, &interrupt);
    }
}