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

[Takashi Iwai]

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@xxxxxxxxxxxxxxx>
> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@xxxxxxxxxx>
> ---
> 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://lore.kernel.org/lkml/20231016151000.GE119987@fedora/t/
> 
>  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.


thanks,

Takashi