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Date:   Tue, 31 Oct 2023 17:35:42 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     "Michael S. Tsirkin" <mst@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 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@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

Yeah, I see, thanks for sharing!

Stefano