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

[Stefano Garzarella]

On Fri, Oct 27, 2023 at 10:10:30AM -0400, Michael S. Tsirkin wrote:
> On Fri, Oct 27, 2023 at 12:18:00PM +0200, Stefano Garzarella wrote:
> > 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
>
>
> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Fixes: de3a9980d8c3 ("ALSA: virtio: add virtio sound driver")
>
>
> The patch is too big for stable - more than 100 lines added. See:
> Documentation/process/stable-kernel-rules.rst

Yeah, I see, thanks for sharing!

Stefano