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SubjectRe: [PATCH v3] ALSA: virtio: use ack callback
From
Hi Matias,

Thanks for the new patch!



On 24.10.2023 00:06, Matias Ezequiel Vara Larsen wrote:
> This commit uses the ack() callback to determine when a buffer has been
> updated, then exposes it to guest.
>
> The current mechanism splits a dma buffer into descriptors that are
> exposed to the device. This dma buffer is shared with the user
> application. When the device consumes a buffer, the driver moves the
> request from the used ring to available ring.
>
> The driver exposes the buffer to the device without knowing if the
> content has been updated from the user. The section 2.8.21.1 of the
> virtio spec states that: "The device MAY access the descriptor chains
> the driver created and the memory they refer to immediately". If the
> device picks up buffers from the available ring just after it is
> notified, it happens that the content may be old.
>
> When the ack() callback is invoked, the driver exposes only the buffers
> that have already been updated, i.e., enqueued in the available ring.
> Thus, the device always picks up a buffer that is updated.
>
> For capturing, the driver starts by exposing all the available buffers
> to device. After device updates the content of a buffer, it enqueues it
> in the used ring. It is only after the ack() for capturing is issued
> that the driver re-enqueues the buffer in the available ring.
>
> Co-developed-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
> Signed-off-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>
> ---
> Changelog:
> v2 -> v3:
> * Use ack() callback instead of copy()/fill_silence()
> * Change commit name
> * Rewrite commit message
> * Remove virtsnd_pcm_msg_send_locked()
> * Use single callback for both capture and playback
> * Fix kernel test robot warnings regarding documentation
> * v2 patch at:
> https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flore.kernel.org%2flkml%2f87y1fzkq8c.wl%2dtiwai%40suse.de%2fT%2f&umid=6a6586e6-1bcb-48d2-8c0c-75ef6bb15df9&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-090fe82db9a03f0dc8c4f214d4d2e2bf916e7f1f
> v1 -> v2:
> * Use snd_pcm_set_managed_buffer_all()for buffer allocation/freeing.
> * Make virtsnd_pcm_msg_send() generic by specifying the offset and size
> for the modified part of the buffer; this way no assumptions need to
> be made.
> * Disable SNDRV_PCM_INFO_NO_REWINDS since now only sequential
> reading/writing of frames is supported.
> * Correct comment at virtsnd_pcm_msg_send().
> * v1 patch at:
> https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flore.kernel.org%2flkml%2f20231016151000.GE119987%40fedora%2ft%2f&umid=6a6586e6-1bcb-48d2-8c0c-75ef6bb15df9&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-2d4d809544c877beff1a631a29db01290c65d879
>
> sound/virtio/virtio_pcm.c | 1 +
> sound/virtio/virtio_pcm.h | 6 ++-
> sound/virtio/virtio_pcm_msg.c | 80 ++++++++++++++++++++---------------
> sound/virtio/virtio_pcm_ops.c | 30 +++++++++++--
> 4 files changed, 79 insertions(+), 38 deletions(-)
>
> diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
> index c10d91fff2fb..9cc5a95b4913 100644
> --- a/sound/virtio/virtio_pcm.c
> +++ b/sound/virtio/virtio_pcm.c
> @@ -124,6 +124,7 @@ static int virtsnd_pcm_build_hw(struct virtio_pcm_substream *vss,
> values = le64_to_cpu(info->formats);
>
> vss->hw.formats = 0;
> + vss->appl_ptr = 0;
>
> for (i = 0; i < ARRAY_SIZE(g_v2a_format_map); ++i)
> if (values & (1ULL << i)) {
> diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
> index 062eb8e8f2cf..ea3c2845ae9b 100644
> --- a/sound/virtio/virtio_pcm.h
> +++ b/sound/virtio/virtio_pcm.h
> @@ -27,6 +27,7 @@ struct virtio_pcm_msg;
> * substream operators.
> * @buffer_bytes: Current buffer size in bytes.
> * @hw_ptr: Substream hardware pointer value in bytes [0 ... buffer_bytes).
> + * @appl_ptr: Substream application pointer value in bytes [0 ... buffer_bytes).
> * @xfer_enabled: Data transfer state (0 - off, 1 - on).
> * @xfer_xrun: Data underflow/overflow state (0 - no xrun, 1 - xrun).
> * @stopped: True if the substream is stopped and must be released on the device
> @@ -51,13 +52,13 @@ struct virtio_pcm_substream {
> spinlock_t lock;
> size_t buffer_bytes;
> size_t hw_ptr;
> + size_t appl_ptr;
> bool xfer_enabled;
> bool xfer_xrun;
> bool stopped;
> bool suspended;
> struct virtio_pcm_msg **msgs;
> unsigned int nmsgs;
> - int msg_last_enqueued;
> unsigned int msg_count;
> wait_queue_head_t msg_empty;
> };
> @@ -117,7 +118,8 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
>
> void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss);
>
> -int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss);
> +int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss, unsigned long offset,
> + unsigned long bytes);
>
> unsigned int virtsnd_pcm_msg_pending_num(struct virtio_pcm_substream *vss);
>
> diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
> index aca2dc1989ba..106e8e847746 100644
> --- a/sound/virtio/virtio_pcm_msg.c
> +++ b/sound/virtio/virtio_pcm_msg.c
> @@ -155,7 +155,6 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
> sizeof(msg->xfer));
> sg_init_one(&msg->sgs[PCM_MSG_SG_STATUS], &msg->status,
> sizeof(msg->status));
> - msg->length = period_bytes;
> virtsnd_pcm_sg_from(&msg->sgs[PCM_MSG_SG_DATA], sg_num, data,
> period_bytes);
>
> @@ -181,66 +180,81 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
>
> vss->msgs = NULL;
> vss->nmsgs = 0;
> + vss->appl_ptr = 0;
> }
>
> /**
> * virtsnd_pcm_msg_send() - Send asynchronous I/O messages.
> * @vss: VirtIO PCM substream.
> + * @offset: starting position that has been updated
> + * @bytes: number of bytes that has been updated
> *
> * All messages are organized in an ordered circular list. Each time the
> * function is called, all currently non-enqueued messages are added to the
> - * virtqueue. For this, the function keeps track of two values:
> - *
> - * msg_last_enqueued = index of the last enqueued message,
> - * msg_count = # of pending messages in the virtqueue.
> + * virtqueue. For this, the function uses offset and bytes to calculate the
> + * messages that need to be added.
> *
> * Context: Any context. Expects the tx/rx queue and the VirtIO substream
> * spinlocks to be held by caller.
> * Return: 0 on success, -errno on failure.
> */
> -int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
> +int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss, unsigned long offset,
> + unsigned long bytes)
> {
> - struct snd_pcm_runtime *runtime = vss->substream->runtime;
> struct virtio_snd *snd = vss->snd;
> struct virtio_device *vdev = snd->vdev;
> struct virtqueue *vqueue = virtsnd_pcm_queue(vss)->vqueue;
> - int i;
> - int n;
> + unsigned long period_bytes = snd_pcm_lib_period_bytes(vss->substream);
> + unsigned long start, end, i;
> + unsigned int msg_count = vss->msg_count;
> bool notify = false;
> + int rc;
>
> - i = (vss->msg_last_enqueued + 1) % runtime->periods;
> - n = runtime->periods - vss->msg_count;
> + start = offset / period_bytes;
> + end = (offset + bytes - 1) / period_bytes;
>
> - for (; n; --n, i = (i + 1) % runtime->periods) {
> + for (i = start; i <= end; i++) {
> struct virtio_pcm_msg *msg = vss->msgs[i];
> struct scatterlist *psgs[] = {
> &msg->sgs[PCM_MSG_SG_XFER],
> &msg->sgs[PCM_MSG_SG_DATA],
> &msg->sgs[PCM_MSG_SG_STATUS]
> };
> - int rc;
> -
> - msg->xfer.stream_id = cpu_to_le32(vss->sid);
> - memset(&msg->status, 0, sizeof(msg->status));
> -
> - if (vss->direction == SNDRV_PCM_STREAM_PLAYBACK)
> - rc = virtqueue_add_sgs(vqueue, psgs, 2, 1, msg,
> - GFP_ATOMIC);
> - else
> - rc = virtqueue_add_sgs(vqueue, psgs, 1, 2, msg,
> - GFP_ATOMIC);
> -
> - if (rc) {
> - dev_err(&vdev->dev,
> - "SID %u: failed to send I/O message\n",
> - vss->sid);
> - return rc;
> + unsigned long n;
> +
> + n = period_bytes - (offset % period_bytes);
> + if (n > bytes)
> + n = bytes;
> +
> + msg->length += n;
> + if (msg->length == period_bytes) {
> + msg->xfer.stream_id = cpu_to_le32(vss->sid);
> + memset(&msg->status, 0, sizeof(msg->status));
> +
> + if (vss->direction == SNDRV_PCM_STREAM_PLAYBACK)
> + rc = virtqueue_add_sgs(vqueue, psgs, 2, 1, msg,
> + GFP_ATOMIC);
> + else
> + rc = virtqueue_add_sgs(vqueue, psgs, 1, 2, msg,
> + GFP_ATOMIC);
> +
> + if (rc) {
> + dev_err(&vdev->dev,
> + "SID %u: failed to send I/O message\n",
> + vss->sid);
> + return rc;
> + }
> +
> + vss->msg_count++;
> }
>
> - vss->msg_last_enqueued = i;
> - vss->msg_count++;
> + offset = 0;
> + bytes -= n;
> }
>
> + if (msg_count == vss->msg_count)
> + return 0;
> +
> if (!(vss->features & (1U << VIRTIO_SND_PCM_F_MSG_POLLING)))
> notify = virtqueue_kick_prepare(vqueue);
>
> @@ -309,6 +323,8 @@ static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg,
> if (vss->hw_ptr >= vss->buffer_bytes)
> vss->hw_ptr -= vss->buffer_bytes;
>
> + msg->length = 0;
> +
> vss->xfer_xrun = false;
> vss->msg_count--;
>
> @@ -320,8 +336,6 @@ static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg,
> le32_to_cpu(msg->status.latency_bytes));
>
> schedule_work(&vss->elapsed_period);
> -
> - virtsnd_pcm_msg_send(vss);
> } else if (!vss->msg_count) {
> wake_up_all(&vss->msg_empty);
> }
> diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
> index f8bfb87624be..21cde37ecfa3 100644
> --- a/sound/virtio/virtio_pcm_ops.c
> +++ b/sound/virtio/virtio_pcm_ops.c
> @@ -282,7 +282,6 @@ static int virtsnd_pcm_prepare(struct snd_pcm_substream *substream)
>
> vss->buffer_bytes = snd_pcm_lib_buffer_bytes(substream);
> vss->hw_ptr = 0;
> - vss->msg_last_enqueued = -1;
> } else {
> struct snd_pcm_runtime *runtime = substream->runtime;
> unsigned int buffer_bytes = snd_pcm_lib_buffer_bytes(substream);
> @@ -324,7 +323,7 @@ static int virtsnd_pcm_trigger(struct snd_pcm_substream *substream, int command)
> struct virtio_snd_queue *queue;
> struct virtio_snd_msg *msg;
> unsigned long flags;
> - int rc;
> + int rc = 0;
>
> switch (command) {
> case SNDRV_PCM_TRIGGER_START:
> @@ -333,7 +332,8 @@ static int virtsnd_pcm_trigger(struct snd_pcm_substream *substream, int command)
>
> spin_lock_irqsave(&queue->lock, flags);
> spin_lock(&vss->lock);
> - rc = virtsnd_pcm_msg_send(vss);
> + if (vss->direction == SNDRV_PCM_STREAM_CAPTURE)
> + rc = virtsnd_pcm_msg_send(vss, 0, vss->buffer_bytes);
> if (!rc)
> vss->xfer_enabled = true;
> spin_unlock(&vss->lock);
> @@ -450,6 +450,29 @@ virtsnd_pcm_pointer(struct snd_pcm_substream *substream)
> return hw_ptr;
> }
>
> +static int virtsnd_pcm_ack(struct snd_pcm_substream *substream)
> +{
> + struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
> + struct virtio_snd_queue *queue = virtsnd_pcm_queue(vss);
> + unsigned long flags;
> + struct snd_pcm_runtime *runtime = vss->substream->runtime;
> + ssize_t appl_pos = frames_to_bytes(runtime, runtime->control->appl_ptr);
> + ssize_t buf_size = frames_to_bytes(runtime, runtime->buffer_size);
> + int rc;
> +
> + spin_lock_irqsave(&queue->lock, flags);
> + spin_lock(&vss->lock);
> +
> + ssize_t bytes = (appl_pos - vss->appl_ptr) % buf_size;
> +
> + rc = virtsnd_pcm_msg_send(vss, vss->appl_ptr % buf_size, bytes);
> + vss->appl_ptr += bytes;

I think it makes sense to store vss->appl_ptr in frames (type
snd_pcm_uframes_t instead of size_t), and do all calculations in frames.
You could do convertion before invoking virtsnd_pcm_msg_send().

We also need to either disable rewinds (SNDRV_PCM_INFO_NO_REWINDS), or take
into account the case when the new runtime->control->appl_ptr value is less
than the old one.


Best regards,

> +
> + spin_unlock(&vss->lock);
> + spin_unlock_irqrestore(&queue->lock, flags);
> + return rc;
> +}
> +
> /* PCM substream operators map. */
> const struct snd_pcm_ops virtsnd_pcm_ops = {
> .open = virtsnd_pcm_open,
> @@ -461,4 +484,5 @@ const struct snd_pcm_ops virtsnd_pcm_ops = {
> .trigger = virtsnd_pcm_trigger,
> .sync_stop = virtsnd_pcm_sync_stop,
> .pointer = virtsnd_pcm_pointer,
> + .ack = virtsnd_pcm_ack,
> };
>
> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa

--
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin

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