Re: [RFC PATCH v3 1/2] drm: Add generic colorkey properties for DRM planes

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Jul 06, 2018 at 05:58:50PM +0300, Dmitry Osipenko wrote:
> On Friday, 6 July 2018 17:10:10 MSK Ville Syrjälä wrote:
> > IIRC my earlier idea was to have different colorkey modes for the
> > min+max and value+mask modes. That way userspace might actually have
> > some chance of figuring out which bits of state actually do something.
> > Although for Intel hw I think the general rule is that min+max for YUV,
> > value+mask for RGB, so it's still not 100% clear what to pick if the
> > plane supports both.
> > 
> > I guess one alternative would be to have min+max only, and the driver
> > would reject 'min != max' if it only uses a single value?
> > 
> 
> You should pick both and reject unsupported property values based on the 
> planes framebuffer format. So it will be possible to set unsupported values 
> while plane is disabled because it doesn't have an associated framebuffer and 
> then atomic check will fail to enable plane if property values are invalid for 
> the given format.

The colorkey which is attached to a plane 'A' is not applied to plane
'A', so the format of plane 'A' is not relevant.  The colorkey is
applied to some other plane which will be below this plane in terms
of the plane blending operation.

What if you have several planes below plane 'A' with differing
framebuffer formats - maybe an ARGB8888 plane and a ARGB1555 plane -
do you decide to limit the colorkey to 8bits per channel, or to
ARGB1555 format?

The answer is, of course, hardware dependent - generic code can't
know the details of the colorkey implementation, which could be one
of:

  lower plane data -> expand to 8bpc -> match ARGB8888 colorkey
  lower plane data -> match ARGB8888 reduced to plane compatible colorkey

which will give different results depending on the format of the
lower plane data.

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