Re: [RFC PATCH] ALSA: virtio: use copy and fill_silence callbacks

[Takashi Iwai]

On Mon, 16 Oct 2023 17:10:00 +0200,
Stefan Hajnoczi wrote:
> 
> On Thu, Oct 12, 2023 at 05:10:50PM +0200, Matias Ezequiel Vara Larsen wrote:
> > This commit replaces the mmap mechanism with the copy() and
> > fill_silence() callbacks for both capturing and playback for the
> > virtio-sound driver. This change is required to prevent the updating of
> > the content of a buffer that is already in the available ring.
> > 
> > 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.
> > 
> > By providing the copy() callback, the driver first updates the content
> > of the buffer, and then, exposes the buffer to the device by enqueuing
> > it in the available ring. Thus, 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 copy() for capturing is issued
> > that the driver re-enqueues the buffer in the available ring.
> > 
> > Note that the copy() function assumes that user is always writing a
> > period. Testing shows that this is true but I may be wrong. This RFC
> > aims at clarifying this.
> 
> This sounds like an ALSA driver API question.
> 
> Jaroslav and Takashi: Any thoughts?

The size of the transfer is determined solely by PCM appl_ptr and
hw_ptr values.  The former is what applications update via the mmap
commit or the normal read/write operations, while the latter is
updated via PCM position() op.  So basically the granularity depends
on the values returned from PCM position op.


Takashi

> 
> Stefan
> 
> > 
> > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@xxxxxxxxxx>
> > ---
> >  sound/virtio/virtio_pcm.c     | 11 ++--
> >  sound/virtio/virtio_pcm.h     |  9 +++-
> >  sound/virtio/virtio_pcm_msg.c | 50 ++++++++++++++++---
> >  sound/virtio/virtio_pcm_ops.c | 94 +++++++++++++++++++++++++++++++----
> >  4 files changed, 137 insertions(+), 27 deletions(-)
> > 
> > diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
> > index c10d91fff2fb..bfe982952303 100644
> > --- a/sound/virtio/virtio_pcm.c
> > +++ b/sound/virtio/virtio_pcm.c
> > @@ -104,8 +104,6 @@ static int virtsnd_pcm_build_hw(struct virtio_pcm_substream *vss,
> >  	 * only message-based transport.
> >  	 */
> >  	vss->hw.info =
> > -		SNDRV_PCM_INFO_MMAP |
> > -		SNDRV_PCM_INFO_MMAP_VALID |
> >  		SNDRV_PCM_INFO_BATCH |
> >  		SNDRV_PCM_INFO_BLOCK_TRANSFER |
> >  		SNDRV_PCM_INFO_INTERLEAVED |
> > @@ -471,12 +469,11 @@ int virtsnd_pcm_build_devs(struct virtio_snd *snd)
> >  			for (kss = ks->substream; kss; kss = kss->next)
> >  				vs->substreams[kss->number]->substream = kss;
> >  
> > -			snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_ops);
> > +			if (i == SNDRV_PCM_STREAM_CAPTURE)
> > +				snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_capture_ops);
> > +			else
> > +				snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_playback_ops);
> >  		}
> > -
> > -		snd_pcm_set_managed_buffer_all(vpcm->pcm,
> > -					       SNDRV_DMA_TYPE_VMALLOC, NULL,
> > -					       0, 0);
> >  	}
> >  
> >  	return 0;
> > diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
> > index 062eb8e8f2cf..1c1106ec971f 100644
> > --- a/sound/virtio/virtio_pcm.h
> > +++ b/sound/virtio/virtio_pcm.h
> > @@ -50,6 +50,8 @@ struct virtio_pcm_substream {
> >  	struct work_struct elapsed_period;
> >  	spinlock_t lock;
> >  	size_t buffer_bytes;
> > +	u8 *buffer;
> > +	size_t buffer_sz;
> >  	size_t hw_ptr;
> >  	bool xfer_enabled;
> >  	bool xfer_xrun;
> > @@ -90,7 +92,8 @@ struct virtio_pcm {
> >  	struct virtio_pcm_stream streams[SNDRV_PCM_STREAM_LAST + 1];
> >  };
> >  
> > -extern const struct snd_pcm_ops virtsnd_pcm_ops;
> > +extern const struct snd_pcm_ops virtsnd_pcm_playback_ops;
> > +extern const struct snd_pcm_ops virtsnd_pcm_capture_ops;
> >  
> >  int virtsnd_pcm_validate(struct virtio_device *vdev);
> >  
> > @@ -117,7 +120,9 @@ 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, bool single);
> > +
> > +int virtsnd_pcm_msg_send_locked(struct virtio_pcm_substream *vss, bool single);
> >  
> >  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..9a5f9814cb62 100644
> > --- a/sound/virtio/virtio_pcm_msg.c
> > +++ b/sound/virtio/virtio_pcm_msg.c
> > @@ -132,7 +132,6 @@ static void virtsnd_pcm_sg_from(struct scatterlist *sgs, int nsgs, u8 *data,
> >  int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
> >  			  unsigned int periods, unsigned int period_bytes)
> >  {
> > -	struct snd_pcm_runtime *runtime = vss->substream->runtime;
> >  	unsigned int i;
> >  
> >  	vss->msgs = kcalloc(periods, sizeof(*vss->msgs), GFP_KERNEL);
> > @@ -142,7 +141,7 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
> >  	vss->nmsgs = periods;
> >  
> >  	for (i = 0; i < periods; ++i) {
> > -		u8 *data = runtime->dma_area + period_bytes * i;
> > +		u8 *data = vss->buffer + period_bytes * i;
> >  		int sg_num = virtsnd_pcm_sg_num(data, period_bytes);
> >  		struct virtio_pcm_msg *msg;
> >  
> > @@ -186,10 +185,12 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
> >  /**
> >   * virtsnd_pcm_msg_send() - Send asynchronous I/O messages.
> >   * @vss: VirtIO PCM substream.
> > + * @single: true to enqueue a single message, false to enqueue all of them.
> >   *
> >   * 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:
> > + * function is called, first non-enqueued message is added to the virtqueue.
> > + * When single is True, only the first message is enqueued. When False, all the
> > + * available messages are enqueued.  The function keeps track of two values:
> >   *
> >   *   msg_last_enqueued = index of the last enqueued message,
> >   *   msg_count = # of pending messages in the virtqueue.
> > @@ -198,7 +199,7 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
> >   *          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, bool single)
> >  {
> >  	struct snd_pcm_runtime *runtime = vss->substream->runtime;
> >  	struct virtio_snd *snd = vss->snd;
> > @@ -211,6 +212,13 @@ int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
> >  	i = (vss->msg_last_enqueued + 1) % runtime->periods;
> >  	n = runtime->periods - vss->msg_count;
> >  
> > +	if (single) {
> > +		if (n < 1)
> > +			return -EFAULT;
> > +
> > +		n = 1;
> > +	}
> > +
> >  	for (; n; --n, i = (i + 1) % runtime->periods) {
> >  		struct virtio_pcm_msg *msg = vss->msgs[i];
> >  		struct scatterlist *psgs[] = {
> > @@ -250,6 +258,36 @@ int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
> >  	return 0;
> >  }
> >  
> > +/**
> > + * virtsnd_pcm_msg_send_locked() - Send asynchronous I/O messages.
> > + * @vss: VirtIO PCM substream.
> > + * @single: true to enqueue a single message, false to enqueue all of them.
> > + *
> > + * This function holds the tx/rx queue and the VirtIO substream spinlocks
> > + * before calling virtsnd_pcm_msg_send(). This is a wrapper function to ease
> > + * the invocation of virtsnd_pcm_msg_send().
> > + *
> > + * Context: Any context.
> > + * Return: 0 on success, -errno on failure.
> > + */
> > +
> > +int virtsnd_pcm_msg_send_locked(struct virtio_pcm_substream *vss, bool single)
> > +{
> > +	struct virtio_snd_queue *queue;
> > +	int rc;
> > +	unsigned long flags;
> > +
> > +	queue = virtsnd_pcm_queue(vss);
> > +
> > +	spin_lock_irqsave(&queue->lock, flags);
> > +	spin_lock(&vss->lock);
> > +	rc = virtsnd_pcm_msg_send(vss, single);
> > +	spin_unlock(&vss->lock);
> > +	spin_unlock_irqrestore(&queue->lock, flags);
> > +
> > +	return rc;
> > +}
> > +
> >  /**
> >   * virtsnd_pcm_msg_pending_num() - Returns the number of pending I/O messages.
> >   * @vss: VirtIO substream.
> > @@ -320,8 +358,6 @@ 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..a208439dbff8 100644
> > --- a/sound/virtio/virtio_pcm_ops.c
> > +++ b/sound/virtio/virtio_pcm_ops.c
> > @@ -238,6 +238,11 @@ static int virtsnd_pcm_hw_params(struct snd_pcm_substream *substream,
> >  	 */
> >  	virtsnd_pcm_msg_free(vss);
> >  
> > +	vss->buffer_sz = params_buffer_bytes(hw_params);
> > +	vss->buffer = alloc_pages_exact(vss->buffer_sz, GFP_KERNEL);
> > +	if (!vss->buffer)
> > +		return -ENOMEM;
> > +
> >  	return virtsnd_pcm_msg_alloc(vss, params_periods(hw_params),
> >  				     params_period_bytes(hw_params));
> >  }
> > @@ -257,6 +262,11 @@ static int virtsnd_pcm_hw_free(struct snd_pcm_substream *substream)
> >  	if (!virtsnd_pcm_msg_pending_num(vss))
> >  		virtsnd_pcm_msg_free(vss);
> >  
> > +	if (vss->buffer) {
> > +		free_pages_exact(vss->buffer, vss->buffer_sz);
> > +		vss->buffer = NULL;
> > +	}
> > +
> >  	return 0;
> >  }
> >  
> > @@ -331,15 +341,18 @@ static int virtsnd_pcm_trigger(struct snd_pcm_substream *substream, int command)
> >  	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
> >  		queue = virtsnd_pcm_queue(vss);
> >  
> > -		spin_lock_irqsave(&queue->lock, flags);
> > -		spin_lock(&vss->lock);
> > -		rc = virtsnd_pcm_msg_send(vss);
> > -		if (!rc)
> > -			vss->xfer_enabled = true;
> > -		spin_unlock(&vss->lock);
> > -		spin_unlock_irqrestore(&queue->lock, flags);
> > -		if (rc)
> > -			return rc;
> > +		// The buffers should be exposed first during capturing so that
> > +		// the device can consume them. Capturing cannot begin
> > +		// otherwise.
> > +		if (vss->direction == SNDRV_PCM_STREAM_CAPTURE) {
> > +			rc = virtsnd_pcm_msg_send_locked(vss, false);
> > +			if (rc)
> > +				return rc;
> > +		}
> > +
> > +		spin_lock_irqsave(&vss->lock, flags);
> > +		vss->xfer_enabled = true;
> > +		spin_unlock_irqrestore(&vss->lock, flags);
> >  
> >  		msg = virtsnd_pcm_ctl_msg_alloc(vss, VIRTIO_SND_R_PCM_START,
> >  						GFP_KERNEL);
> > @@ -450,8 +463,66 @@ virtsnd_pcm_pointer(struct snd_pcm_substream *substream)
> >  	return hw_ptr;
> >  }
> >  
> > -/* PCM substream operators map. */
> > -const struct snd_pcm_ops virtsnd_pcm_ops = {
> > +static int virtsnd_pcm_pb_copy(struct snd_pcm_substream *substream,
> > +			       int channel, unsigned long pos, struct iov_iter
> > +			       *src, unsigned long count)
> > +{
> > +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
> > +
> > +	if (unlikely(pos + count > vss->buffer_sz))
> > +		return -EINVAL;
> > +
> > +	if (copy_from_iter(vss->buffer + pos, count, src) != count)
> > +		return -EFAULT;
> > +
> > +	return virtsnd_pcm_msg_send_locked(vss, true);
> > +}
> > +
> > +static int virtsnd_pcm_cap_copy(struct snd_pcm_substream *substream,
> > +				int channel, unsigned long pos, struct iov_iter
> > +				*dst, unsigned long count)
> > +{
> > +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
> > +
> > +	if (unlikely(pos + count > vss->buffer_sz))
> > +		return -EINVAL;
> > +
> > +	if (copy_to_iter(vss->buffer + pos, count, dst) != count)
> > +		return -EFAULT;
> > +
> > +	return virtsnd_pcm_msg_send_locked(vss, true);
> > +}
> > +
> > +static int virtsnd_pcm_pb_silence(struct snd_pcm_substream *substream, int channel,
> > +				  unsigned long pos, unsigned long count)
> > +{
> > +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
> > +
> > +	if (unlikely(pos + count > vss->buffer_sz))
> > +		return -EINVAL;
> > +
> > +	memset(vss->buffer + pos, 0, count);
> > +
> > +	return virtsnd_pcm_msg_send_locked(vss, true);
> > +}
> > +
> > +/* PCM substream operators map for playback. */
> > +const struct snd_pcm_ops virtsnd_pcm_playback_ops = {
> > +	.open = virtsnd_pcm_open,
> > +	.close = virtsnd_pcm_close,
> > +	.ioctl = snd_pcm_lib_ioctl,
> > +	.hw_params = virtsnd_pcm_hw_params,
> > +	.hw_free = virtsnd_pcm_hw_free,
> > +	.prepare = virtsnd_pcm_prepare,
> > +	.trigger = virtsnd_pcm_trigger,
> > +	.sync_stop = virtsnd_pcm_sync_stop,
> > +	.pointer = virtsnd_pcm_pointer,
> > +	.copy = virtsnd_pcm_pb_copy,
> > +	.fill_silence = virtsnd_pcm_pb_silence,
> > +};
> > +
> > +/* PCM substream operators map for capturing. */
> > +const struct snd_pcm_ops virtsnd_pcm_capture_ops = {
> >  	.open = virtsnd_pcm_open,
> >  	.close = virtsnd_pcm_close,
> >  	.ioctl = snd_pcm_lib_ioctl,
> > @@ -461,4 +532,5 @@ const struct snd_pcm_ops virtsnd_pcm_ops = {
> >  	.trigger = virtsnd_pcm_trigger,
> >  	.sync_stop = virtsnd_pcm_sync_stop,
> >  	.pointer = virtsnd_pcm_pointer,
> > +	.copy = virtsnd_pcm_cap_copy,
> >  };
> > 
> > base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
> > -- 
> > 2.41.0
> > 
> Verifying...