[PATCH v4] ALSA: virtio: use ack callback

Michael S. Tsirkin
Fri, 27 Oct 2023 14:10:30 -0000

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 at
opensynergy.com>
> > > Signed-off-by: Anton Yakovlev <anton.yakovlev at
opensynergy.com>
> > > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar at
redhat.com>
> >
> > 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 at redhat.com>
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



--
MST