Message-ID: <a7808ac5-3f03-4584-93bc-485d358d57c1@opensynergy.com>
Date:   Tue, 24 Oct 2023 09:11:10 +0900
From:   Anton Yakovlev <anton.yakovlev@...nsynergy.com>
To:     Matias Ezequiel Vara Larsen <mvaralar@...hat.com>, mst@...hat.com
Cc:     perex@...ex.cz, tiwai@...e.com,
        virtualization@...ts.linux-foundation.org,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, stefanha@...hat.com, sgarzare@...hat.com,
        manos.pitsidianakis@...aro.org, mripard@...hat.com
Subject: Re: [PATCH v3] ALSA: virtio: use ack callback

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@...nsynergy.com>
> Signed-off-by: Anton Yakovlev <anton.yakovlev@...nsynergy.com>
> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@...hat.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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