linux-kernel - Re: [PATCHv2 3/3] media: uvcvideo: add UVC 1.5 ROI control

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Message-ID: <YFLbYjm0VyzaEMkr@google.com>
Date:   Thu, 18 Mar 2021 13:47:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Ricardo Ribalda <ribalda@...omium.org>
Subject: Re: [PATCHv2 3/3] media: uvcvideo: add UVC 1.5 ROI control

On (21/03/17 08:58), Ricardo Ribalda Delgado wrote:
[..]
> >
> > GET_CUR?
> yep
> 
> >
> > > https://www.kernel.org/doc/html/v4.13/media/uapi/v4l/vidioc-g-selection.html?highlight=vidioc_s_selection
> > > On success the struct v4l2_rect r field contains the adjusted
> > > rectangle.
> >
> > What is the adjusted rectangle here? Does this mean that firmware can
> > successfully apply SET_CUR and return 0, but in reality it was not happy
> > with the rectangle dimensions so it modified it behind the scenes?
> 
> I can imagine that some hw might have spooky requirements for the roi
> rectangle (multiple of 4, not crossing the bayer filter, odd/even
> line...) so they might be able to go the closest valid config.

Hmm. Honestly, I'm very unsure about it. ROI::SET_CUR can be a very
hot path, depending on what user-space considers to be of interest
and how frequently that object of interest changes its position/shape/etc.
Doing GET_CUR after every SET_CUR doubles the number of firmware calls
we issue, that's for sure; is it worth it - that's something that I'm
not sure of.

May I please ask for more opinions on this?

	-ss