Re: [PATCH v2] ALSA: virtio: use copy and fill_silence callbacks

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

Hello Takashi,

On Thu, Oct 19, 2023 at 09:48:03AM +0200, Takashi Iwai wrote:
> On Thu, 19 Oct 2023 03:20:19 +0200,
> Anton Yakovlev wrote:
> > 
> > Hi Takashi,
> > 
> > On 19.10.2023 03:07, Takashi Iwai wrote:
> > > On Wed, 18 Oct 2023 12:48:23 +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. During copy(), the number of requests enqueued depends on the
> > >> "pos" and "bytes" arguments. The length of each request is period_size
> > >> bytes.
> > >> 
> > >> 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.
> > >> 
> > >> Co-developed-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
> > >> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>
> > >> ---
> > >> Changelog:
> > >> 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=2f305b77-83e7-47b6-a461-a8ca67d0bfe2&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-2d5775265e7e1741ae8eb783a3cb78ed553093c1
> > >> 
> > >>   sound/virtio/virtio_pcm.c     |  7 ++-
> > >>   sound/virtio/virtio_pcm.h     |  9 ++--
> > >>   sound/virtio/virtio_pcm_msg.c | 93 ++++++++++++++++++++++-------------
> > >>   sound/virtio/virtio_pcm_ops.c | 81 +++++++++++++++++++++++++-----
> > >>   4 files changed, 137 insertions(+), 53 deletions(-)
> > > 
> > > Most of the code changes look good, but I wonder:
> > > 
> > >> 
> > >> diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
> > >> index c10d91fff2fb..66d67eef1bcc 100644
> > >> --- a/sound/virtio/virtio_pcm.c
> > >> +++ b/sound/virtio/virtio_pcm.c
> > >> @@ -104,12 +104,11 @@ 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 |
> > > 
> > > Do we need the removal of those MMAP features inevitably?
> > > Usually mmap can still work even if the driver implements the copy
> > > ops.  Those aren't always mutual exclusive.
> > 
> > The driver uses a message queue to communicate with the device. Thus,
> > the audio buffer is sliced into several I/O requests (= number of
> > periods) of the same size (= period size).
> > 
> > Before this, all such requests were enqueued when the substream started,
> > and immediately re-enqueued once the request is completed. This approach
> > made it possible to add mmap support. But for mmap there are no explicit
> > notifications from the application how many frames were written or read.
> > Thus, it was assumed that the virtual device should read/write frames to
> > requests based on timings. And there are some problems here:
> > 
> >   1. This was found to violate the virtio specification: if a request is
> >      already in the queue, the device can safely read/write there at any
> >      time.
> >   2. It looks like this breaks the use case with swiotlb. Personally I'm
> >      not sure how the application handles DMA ownership in the case of
> >      mmaped buffer.
> > 
> > To correctly implement mmap support, instead of transferring data via a
> > message queue, the driver and device must have a shared memory region.
> > We can add mmap in the future when we expand the functionality of the
> > device to support such shared memory.
> 
> Ah, then this implementation might be an overkill.  You're still using
> the (intermediate) vmalloc buffer allocated via PCM managed mode, and
> the actual data is copied from/to there.  So it doesn't conflict with
> the mmap operation at all.
> 
> I guess that the problem you're trying to solve (the immediate data
> transfer to the queue) can be implemented rather via PCM ack callback
> instead.  ALSA PCM core notifies the possible data transfer via PCM
> ack callback right after each change of appl_ptr or hw_ptr, including
> each read/write op or mmap commit.  Then the driver can check the
> change of appl_ptr (or hw_ptr for capture), fetch the newly available
> data, and queue it immediately.
> 
> Usually together with the use of ack callback, the driver sets
> SNDRV_PCM_INFO_SYNC_APPLPTR flag.  This prevents the mmap of the PCM
> control record (not the audio data) and enforces the use of
> SNDRV_PCM_IOCTL_SYNC_PTR ioctl instead (so that the driver always gets
> the ack callback).
> 
> 

Thanks for your comments. If I understand correctly, we have two
options:
1. Use copy/fill_silence callbacks and use my own buffers thus disabling
mmap.
2. Use mmap and the ack callback to track when appl_ptr changes thus
moving the content to the queues after it has been updated.

Am I right? 

Thanks, Matias.

> thanks,
> 
> Takashi
> 
> 
> > 
> > 
> > Best regards,
> > 
> > > 
> > > 
> > > thanks,
> > > 
> > > Takashi
> > 
> > -- 
> > Anton Yakovlev
> > Senior Software Engineer
> > 
> > OpenSynergy GmbH
> > Rotherstr. 20, 10245 Berlin
> > 
> 

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