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Date:   Fri, 27 Oct 2023 12:18:00 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Takashi Iwai <tiwai@suse.de>, mst@redhat.com
Cc:     Matias Ezequiel Vara Larsen <mvaralar@redhat.com>,
        anton.yakovlev@opensynergy.com, mst@redhat.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 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