linux-kernel - Re: [RFC] METADATA design using V4l2 Request API

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Date:   Thu, 28 May 2020 22:18:54 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     dikshita@...eaurora.org, Hans Verkuil <hverkuil@...all.nl>
Cc:     linux-media@...r.kernel.org, stanimir.varbanov@...aro.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        vgarodia@...eaurora.org, majja@...eaurora.org, jdas@...eaurora.org
Subject: Re: [RFC] METADATA design using V4l2 Request API

Le jeudi 28 mai 2020 à 16:18 +0530, dikshita@...eaurora.org a écrit :
> > not allowed. So I need to know more about this.
> > Regards,
> >        Hans
> 
> we need this for use cases like HDR10+ where metadata info is part of
> the bitstream.
> 
> To handle such frame specific data, support for request api on capture 
> plane would be needed.

I have a bit of a mixed impression here. Considering how large the ioctl
interface overhead is, relying on HW parser to extract this medata woud be the
last thing I would consider.

Instead, I'm quite convince we can achieve greater and likely zero-copy
perfromance by locating this header in userspace. So everytime I see this kind
of API, were the HW is *needed* to parse a trivial bit of bistream, I ended
thinking that we simply craft this API to expose this because the HW can do it,
but no further logical thinking or higher level design seems to have been
applied.

I'm sorry for this open critic, but are we designing this because the HW
designer exposed that feature? This is so low complexity to extract in
userspace, with the bonus that we are not forced into expanding the
representation to another form immediatly, as maybe the display will be able to
handle that form directly (where converting to a C structure and then back to
some binary format to satisfy DRM property seems very sub-optimal).

Nicolas