Re: [PATCH v4] ALSA: virtio: use ack callback

[Stefano Garzarella]

On Fri, Oct 27, 2023 at 11:27:40AM +0200, Takashi Iwai wrote:
> On Wed, 25 Oct 2023 11:49:19 +0200,
> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Anton Yakovlev <anton.yakovlev@xxxxxxxxxxxxxxx>
> > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@xxxxxxxxxx>
>
> Applied now to for-next branch.

Cool, thanks for that!

I just wonder if we should CC stable since we are fixing a virtio
specification violation.

@Michael what do you think?

Thanks,
Stefano