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References: <20180110160732.7722-1-gustavo@padovan.org> <20180110160732.7722-2-gustavo@padovan.org>
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From: Alexandre Courbot <acourbot@chromium.org>
Date: Tue, 16 Jan 2018 11:35:33 +0900
Message-ID: <CAPBb6MUNPtD-74ON6Wvw4waOuk4YPmYjXgFV=fBeCKTozwHJjw@mail.gmail.com>
Subject: Re: [PATCH v7 1/6] [media] vb2: add is_unordered callback for drivers
To: Gustavo Padovan <gustavo@padovan.org>
Cc: Linux Media Mailing List <linux-media@vger.kernel.org>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
        Shuah Khan <shuahkh@osg.samsung.com>,
        Pawel Osciak <pawel@osciak.com>,
        Sakari Ailus <sakari.ailus@iki.fi>,
        Brian Starkey <brian.starkey@arm.com>,
        Thierry Escande <thierry.escande@collabora.com>,
        linux-kernel@vger.kernel.org,
        Gustavo Padovan <gustavo.padovan@collabora.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, Jan 15, 2018 at 9:01 PM, Gustavo Padovan <gustavo@padovan.org> wrote:
> 2018-01-15 Alexandre Courbot <acourbot@chromium.org>:
>
>> On Thu, Jan 11, 2018 at 1:07 AM, Gustavo Padovan <gustavo@padovan.org> wrote:
>> > From: Gustavo Padovan <gustavo.padovan@collabora.com>
>> >
>> > Explicit synchronization benefits a lot from ordered queues, they fit
>> > better in a pipeline with DRM for example so create a opt-in way for
>> > drivers notify videobuf2 that the queue is unordered.
>> >
>> > Drivers don't need implement it if the queue is ordered.
>>
>> This is going to make user-space believe that *all* vb2 drivers use
>> ordered queues by default, at least until non-ordered drivers catch up
>> with this change. Wouldn't it be less dangerous to do the opposite
>> (make queues non-ordered by default)?
>
> The rational behind this decision was because most formats/drivers are
> ordered so only a small amount of drivers need to changed. I think this
> was proposed by Hans on the Media Summit.

As long as all concerned drivers are updated we should be on the safe
side. At first I was surprised that we expose the ordering feature in
a negative tense, but if the vast majority of devices are ordered this
probably makes sense.