virtio-sound: implement audio output (TX)

Handle output IO messages in the transmit (TX) virtqueue.

It allocates a VirtIOSoundPCMBuffer for each IO message and copies the
data buffer to it. When the IO buffer is written to the host's sound
card, the guest will be notified that it has been consumed.

The lifetime of an IO message is:

1. Guest sends IO message to TX virtqueue.
2. QEMU adds it to the appropriate stream's IO buffer queue.
3. Sometime later, the host audio backend calls the output callback,
   virtio_snd_pcm_out_cb(), which is defined with an AUD_open_out()
   call. The callback gets an available number of bytes the backend can
   receive. Then it writes data from the IO buffer queue to the backend.
   If at any time a buffer is exhausted, it is returned to the guest as
   completed.
4. If the guest releases the stream, its buffer queue is flushed by
   attempting to write any leftover data to the audio backend and
   releasing all IO messages back to the guest. This is how according to
   the spec the guest knows the release was successful.

Based-on: https://github.com/OpenSynergy/qemu/commit/5a2f350eec5d157b90d9c7b40a8e603f4da92471
Signed-off-by: Igor Skalkin <Igor.Skalkin@opensynergy.com>
Signed-off-by: Anton Yakovlev <Anton.Yakovlev@opensynergy.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <b7c6fc458c763d09a4abbcb620ae9b220afa5b8f.1698062525.git.manos.pitsidianakis@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/include/hw/audio/virtio-snd.h b/include/hw/audio/virtio-snd.h
index cc14c875ed..c3767f442b 100644
--- a/include/hw/audio/virtio-snd.h
+++ b/include/hw/audio/virtio-snd.h
@@ -77,6 +77,50 @@ typedef struct virtio_snd_ctrl_command virtio_snd_ctrl_command;
 
 typedef struct VirtIOSoundPCM VirtIOSoundPCM;
 
+typedef struct VirtIOSoundPCMBuffer VirtIOSoundPCMBuffer;
+
+/*
+ * The VirtIO sound spec reuses layouts and values from the High Definition
+ * Audio spec (virtio/v1.2: 5.14 Sound Device). This struct handles each I/O
+ * message's buffer (virtio/v1.2: 5.14.6.8 PCM I/O Messages).
+ *
+ * In the case of TX (i.e. playback) buffers, we defer reading the raw PCM data
+ * from the virtqueue until QEMU's sound backsystem calls the output callback.
+ * This is tracked by the `bool populated;` field, which is set to true when
+ * data has been read into our own buffer for consumption.
+ *
+ * VirtIOSoundPCMBuffer has a dynamic size since it includes the raw PCM data
+ * in its allocation. It must be initialized and destroyed as follows:
+ *
+ *   size_t size = [[derived from owned VQ element descriptor sizes]];
+ *   buffer = g_malloc0(sizeof(VirtIOSoundPCMBuffer) + size);
+ *   buffer->elem = [[owned VQ element]];
+ *
+ *   [..]
+ *
+ *   g_free(buffer->elem);
+ *   g_free(buffer);
+ */
+struct VirtIOSoundPCMBuffer {
+    QSIMPLEQ_ENTRY(VirtIOSoundPCMBuffer) entry;
+    VirtQueueElement *elem;
+    VirtQueue *vq;
+    size_t size;
+    /*
+     * In TX / Plaback, `offset` represents the first unused position inside
+     * `data`. If `offset == size` then there are no unused data left.
+     */
+    uint64_t offset;
+    /* Used for the TX queue for lazy I/O copy from `elem` */
+    bool populated;
+    /*
+     * VirtIOSoundPCMBuffer is an unsized type because it ends with an array of
+     * bytes. The size of `data` is determined from the I/O message's read-only
+     * or write-only size when allocating VirtIOSoundPCMBuffer.
+     */
+    uint8_t data[];
+};
+
 struct VirtIOSoundPCM {
     VirtIOSound *snd;
     /*
@@ -104,7 +148,10 @@ struct VirtIOSoundPCMStream {
         SWVoiceIn *in;
         SWVoiceOut *out;
     } voice;
+    QemuMutex queue_mutex;
     bool active;
+    QSIMPLEQ_HEAD(, VirtIOSoundPCMBuffer) queue;
+    QSIMPLEQ_HEAD(, VirtIOSoundPCMBuffer) invalid;
 };
 
 /*