linux-kernel - Re: [PATCH v2] ALSA: virtio: use copy and fill

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Message-ID: <e50c5a67-d2b7-4ef1-8aaa-309437fa8cb5@opensynergy.com>
Date:   Thu, 19 Oct 2023 10:20:19 +0900
From:   Anton Yakovlev <anton.yakovlev@...nsynergy.com>
To:     Takashi Iwai <tiwai@...e.de>,
        Matias Ezequiel Vara Larsen <mvaralar@...hat.com>
Cc:     mst@...hat.com, 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 v2] ALSA: virtio: use copy and fill_silence callbacks

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


Best regards,

> 
> 
> thanks,
> 
> Takashi

-- 
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin