Re: [PATCH v3 2/3] drm: Add CRTC background color property (v3)

[Eric Anholt <eric@anholt.net>]

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Brian Starkey <Brian.Starkey@arm.com> writes:

> Hi Matt,
>
> On Thu, Nov 15, 2018 at 02:13:45PM -0800, Matt Roper wrote:
>>Some display controllers can be programmed to present non-black colors
>>for pixels not covered by any plane (or pixels covered by the
>>transparent regions of higher planes).  Compositors that want a UI with
>>a solid color background can potentially save memory bandwidth by
>>setting the CRTC background property and using smaller planes to display
>>the rest of the content.
>>
>>To avoid confusion between different ways of encoding RGB data, we
>>define a standard 64-bit format that should be used for this property's
>>value.  Helper functions and macros are provided to generate and dissect
>>values in this standard format with varying component precision values.
>>
>>v2:
>> - Swap internal representation's blue and red bits to make it easier
>>   to read if printed out.  (Ville)
>> - Document bgcolor property in drm_blend.c.  (Sean Paul)
>> - s/background_color/bgcolor/ for consistency between property name and
>>   value storage field.  (Sean Paul)
>> - Add a convenience function to attach property to a given crtc.
>>
>>v3:
>> - Restructure ARGB component extraction macros to be easier to
>>   understand and enclose the parameters in () to avoid calculations
>>   if expressions are passed.  (Sean Paul)
>> - s/rgba/argb/ in helper function/macro names.  Even though the idea is
>>   to not worry about the internal representation of the u64, it can
>>   still be confusing to look at code that uses 'rgba' terminology, but
>>   stores values with argb ordering.  (Ville)
>>
>>Cc: dri-devel@lists.freedesktop.org
>>Cc: wei.c.li@intel.com
>>Cc: harish.krupo.kps@intel.com
>>Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>Cc: Sean Paul <sean@poorly.run>
>>Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
>>Reviewed-by(v2): Sean Paul <sean@poorly.run>
>>---

>
>>+ *	Background color is setup with drm_crtc_add_bgcolor_property().  It
>>+ *	controls the RGB color of a full-screen, fully-opaque layer that exists
>>+ *	below all planes.  This color will be used for pixels not covered by
>>+ *	any plane and may also be blended with plane contents as allowed by a
>>+ *	plane's alpha values.  The background color defaults to black, and is
>>+ *	assumed to be black for drivers that do not expose this property.
>
> Might be worth explicitly mentioning that this value is used as-is,
> without any gamma/gamut conversion before blending, i.e. it's assumed
> to already be in whatever blend-space the CRTC is using (at least I
> assume that's the case?). As we start making the color pipe more
> complicated/explicit, the details matter more.

Raspberry Pi should be able to do background color as well, but the
question of where this property is in the pipeline (particularly for CTM
and gamma) also came up for me.  My background color would apply at the
same stage as plane composition, so before gamma and CTM.

FWIW, my background color on the writeback connector (the only one where
the value would be relevant) can have alpha.

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