linux-kernel - Re: [PATCHv3 5/6] media: uvcvideo: add UVC 1.5 ROI control

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Date:   Wed, 24 Mar 2021 11:52:33 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 5/6] media: uvcvideo: add UVC 1.5 ROI control

On (21/03/24 11:34), Tomasz Figa wrote:
> On Wed, Mar 24, 2021 at 11:31 AM Sergey Senozhatsky
> <senozhatsky@...omium.org> wrote:
[..]
> > > Adjusting the rectangle to something supported by the hardware is
> > > mentioned explicitly in the V4L2 API documentation and is what drivers
> > > have to implement. Returning an error on invalid value is not a
> > > correct behavior here (and similarly for many other operations, e.g.
> > > S_FMT).
> >
> > Well, in this particular case we are talking about user-space that wants
> > to set ROI rectangle that is knowingly violates device's GET_MAX and
> > overflows UVC ROI rectangle u16 value range. That's a clear bug in user-space.
> > Do we want to pretend that user-space does the correct thing and fixup
> > stuff behind the scenes?
> >
> 
> That's how the API is defined. There is a valid use case for this -
> you don't need to run QUERY_CTRL if all you need is setting the
> biggest possible rectangle, just set it to (0, 0), (INT_MAX, INT_MAX).

I guess in our case we need to talk about rectangle,auto-controls tuple
that we send to firmware

	rect {
		(0, 0), (INT_MAX, INT_MAX)
	}
	auto-controls {
		INT_MAX
	}

For ROI user-space also must provide valid auto-controls value, which
normally requires GET_MIN/GET_MAX discovery.

v4l2 selection API mentions only rectangle adjustments and errnos like
-ERANGE also mention "It is not possible to adjust struct v4l2_rect r
rectangle to satisfy all constraints given in the flags argument".

So in case when auto-controls is out of supported range (out of
GET_MIN, GET_MAX range) there is no way for us to tell user-space that
auto-controls is wrong. We probably need silently pick up the first
supported value, but not sure how well this will work out in the end.