linux-kernel - Re: [PATCH v4] ALSA: virtio: use ack callback

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Date:   Fri, 27 Oct 2023 11:27:40 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Matias Ezequiel Vara Larsen <mvaralar@...hat.com>
Cc:     anton.yakovlev@...nsynergy.com, mst@...hat.com, perex@...ex.cz,
        tiwai@...e.com, virtualization@...ts.linux-foundation.org,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, stefanha@...hat.com, sgarzare@...hat.com,
        manos.pitsidianakis@...aro.org, mripard@...hat.com
Subject: Re: [PATCH v4] ALSA: virtio: use ack callback

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@...nsynergy.com>
> Signed-off-by: Anton Yakovlev <anton.yakovlev@...nsynergy.com>
> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@...hat.com>

Applied now to for-next branch.


thanks,

Takashi