linux-kernel - Re: [PATCH v10 01/16] videobuf2: Make struct vb2

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Message-ID: <d72a1651f7b00c06dfcd8e8973293b93298a20ca.camel@collabora.com>
Date:   Tue, 29 May 2018 10:17:25 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     sathyam panda <panda.sathyam9@...il.com>
Cc:     linux-media@...r.kernel.org, kernel@...labora.com,
        Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Pawel Osciak <pawel@...iak.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        Sakari Ailus <sakari.ailus@....fi>,
        Brian Starkey <brian.starkey@....com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>
Subject: Re: [PATCH v10 01/16] videobuf2: Make struct vb2_buffer refcounted

On Fri, 2018-05-25 at 12:11 +0530, sathyam panda wrote:
> Hello,
> 
> On 5/21/18, Ezequiel Garcia <ezequiel@...labora.com> wrote:
> > The in-fence implementation involves having a per-buffer fence callback,
> > that triggers on the fence signal. The fence callback is called
> > asynchronously
> > and needs a valid reference to the associated ideobuf2 buffer.
> > 
> > Allow this by making the vb2_buffer refcounted, so it can be passed
> > to other contexts.
> > 
> 
> -Is it really required, because when a queued buffer with an in_fence
> is deallocated, firstly queue is cancelled.
> -And __vb2_dqbuf is called which calls dma_fence_remove_callback.
> -So if fence callback has been called -__vb2_dqbuf will wait to
> acquire fence lock.
> -So during execution of fence callback, buffers and queue are still valid.
> -And if __vb2_dqbuf remove callback first ,then dma_fence_signal will
> wait for lock
> - so there won't be any fence callback to call for that buffer when
> dma_fence_signal resumes.
> 

Hi Sathyam,

Thanks for your review! The refcount is definitely required,
as the fence callback only schedules a workqueue, which is
completely asynchronous with respect to the rest of the
ioctls.

In particular, the workqueue is not synchronized with
vb2_core_queue_release.

Also, another subtle detail, dma_fence_remove_callback
can fail to remove the callback.

Thanks,
Eze