Re: [PATCH] media: Add support for arbitrary resolution for the ov5642 camera driver

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Hi Mauro,

Could you please comment on this ? In a nutshell (and from my biased point of 
view), the question is "can cropping be configured using S_FMT instead of 
S_CROP ?". The answer is of course no :-)

On Tuesday 30 August 2011 15:13:25 Guennadi Liakhovetski wrote:
> On Tue, 30 Aug 2011, Laurent Pinchart wrote:
> > On Tuesday 30 August 2011 10:55:08 Guennadi Liakhovetski wrote:
> > > On Mon, 29 Aug 2011, Laurent Pinchart wrote:
> > > > On Monday 29 August 2011 14:34:53 Guennadi Liakhovetski wrote:
> > > > > On Mon, 29 Aug 2011, Laurent Pinchart wrote:
> > > > > > On Monday 29 August 2011 14:18:50 Guennadi Liakhovetski wrote:
> > > > > > > On Sun, 28 Aug 2011, Laurent Pinchart wrote:
> > > > > > > 
> > > > > > > [snip]
> > > > > > > 
> > > > > > > > > @@ -774,17 +839,27 @@ static int ov5642_s_fmt(struct
> > > > > > > > > v4l2_subdev *sd,
> > > > > > > > > 
> > > > > > > > >  	ov5642_try_fmt(sd, mf);
> > > > > > > > > 
> > > > > > > > > +	priv->out_size.width		= mf->width;
> > > > > > > > > +	priv->out_size.height		= mf->height;
> > > > > > > > 
> > > > > > > > It looks like to me (but I may be wrong) that you achieve
> > > > > > > > different resolutions using cropping, not scaling. If that's
> > > > > > > > correct you should implement s_crop support and refuse
> > > > > > > > changing the resolution through s_fmt.
> > > > > > > 
> > > > > > > As the patch explains (I think) on several occasions, currently
> > > > > > > only the 1:1 scale is supported, and it was our deliberate
> > > > > > > choice to implement this using the scaling API
> > > > > > 
> > > > > > If you implement cropping, you should use the crop API, not the
> > > > > > scaling API
> > > > > > 
> > > > > > :-)
> > > > > 
> > > > > It's changing both - input and output sizes.
> > > > 
> > > > Sure, but it's still cropping.
> > > 
> > > Why? Isn't it a matter of the PoV?
> > 
> > No it isn't. Cropping is cropping, regardless of how you look at it.
> > 
> > > It changes the output window, i.e., implements S_FMT. And S_FMT is by
> > > far more important / widely used than S_CROP. Refusing to change the
> > > output window and always just returning the == crop size wouldn't be
> > > polite, IMHO.
> > 
> > If your sensor has no scaler the output size is equal to the crop
> > rectangle. There's no way around that, and there's no reason to have the
> > driver behave otherwise.
> > 
> > > Don't think many users would guess to use S_CROP.
> > 
> > Users who want to crop use S_CROP.
> > 
> > > Standard applications a la mplayer don't use S_CROP at all.
> > 
> > That's because they don't want to crop. mplayer expects the driver to
> > perform scaling when it calls S_FMT, and users won't be happy if you
> > crop instead.
> 
> So, here's my opinion, based on the V4L2 spec. I'm going to base on this
> and pull this patch into my tree and let Mauro decide, unless he expresses
> his negative opinion before that.
> 
> The spec defines S_FMT as an operation to set the output (in case of a
> capture device) frame format. Which this driver clearly does. The output
> format should be set, using scaling, however, if the driver or the
> hardware are unable to preserve the exact same input rectangle to satisfy
> the request, the driver is also allowed to change the cropping rectangle
> _as much as necessary_ - S_FMT takes precedence. This has been discussed
> before, and the conclusion was - of the two geometry calls (S_FMT and
> S_CROP) the last call overrides previous setting of the opposite geometry.
> 
> It also defines S_CROP as an operation to set the cropping rectangle. The
> driver is also allowed to change the output window, if it cannot be
> preserved. Similarly, the last call wins.
> 
> Ideally in this situation I would implement both S_CROP and S_FMT and let
> both change the opposite window as needed, which in this case means set it
> equal to the one, being configured. Since most applications are primarily
> interested in the S_FMT call to configure their user interface, I find it
> a wrong approach to refuse S_FMT and always return the current cropping
> rectangle. In such a case the application will possibly be stuck with some
> default output rectangle, because it certainly will _not_ guess to use
> S_CROP to configure it. Whereas if we implement S_FMT with a constant 1:1
> scale the application will get the required UI size. I agree, that
> changing the view area, while changing the output window, is not exactly
> what the user expects, but it's better than presenting all applications
> with a fixed, possibly completely unsuitable, UI window.

-- 
Regards,

Laurent Pinchart
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