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Date:   Fri, 27 Oct 2023 10:10:30 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     Takashi Iwai <tiwai@suse.de>,
        Matias Ezequiel Vara Larsen <mvaralar@redhat.com>,
        anton.yakovlev@opensynergy.com, perex@perex.cz, tiwai@suse.com,
        virtualization@lists.linux-foundation.org,
        alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
        pbonzini@redhat.com, stefanha@redhat.com,
        manos.pitsidianakis@linaro.org, mripard@redhat.com
Subject: Re: [PATCH v4] ALSA: virtio: use ack callback
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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@opensynergy.com>
> > > Signed-off-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
> > > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@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@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