Re: [PATCH v6 2/4] drm: Add CRTC background color property

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

On 1/29/26 11:37 AM, Andy Shevchenko wrote:
> On Thu, Jan 29, 2026 at 02:58:52AM +0200, Cristian Ciocaltea wrote:
>> Some display controllers can be hardware programmed to show non-black
>> colors for pixels that are either not covered by any plane or are
>> exposed through transparent regions of higher planes.  This feature can
>> help reduce memory bandwidth usage, e.g. in compositors managing a UI
>> with a solid background color while using smaller planes to render the
>> remaining content.
>>
>> To support this capability, introduce the BACKGROUND_COLOR standard DRM
>> mode property, which can be attached to a CRTC through the
>> drm_crtc_attach_background_color_property() helper function.
>>
>> Additionally, define a 64-bit ARGB format value to be built with the
>> help of a couple of dedicated DRM_ARGB64_PREP*() helpers.  Individual
>> color components can be extracted with desired precision using the
>> corresponding DRM_ARGB64_GET*() macros.
> 
> ...
> 
>> +/*
>> + * Put 16-bit ARGB values into a standard 64-bit representation that can be
>> + * used for ioctl parameters, inter-driver communication, etc.
>> + *
>> + * If the component values being provided contain less than 16 bits of
>> + * precision, use a conversion ratio to get a better color approximation.
>> + * The ratio is computed as (2^16 - 1) / (2^bpc - 1), where bpc and 16 are
>> + * the input and output precision, respectively.
> 
> Not sure if you should explicitly mention that "bpc must not be 0"
> (it can be derived from the "division by 0" in the given formula,
> but still...).

Comment section updated in v7 [1], though I somehow missed to mention it in the
changelog. :-(

> 
>> + */
>> +#define __DRM_ARGB64_PREP(c, shift)					\
>> +	(((__u64)(c) & __GENMASK(15, 0)) << (shift))
>> +
>> +#define __DRM_ARGB64_PREP_BPC(c, shift, bpc)({				\
> 
> Not sure if this is an accepted style in DRM, by I find it difficult
> to follow. I would expect the "({" be on a separate line.

I initially got confused by the plethora of different styles, e.g. in
"include/linux/math.h" we can find:

  #define rounddown(x, y) (				\
  [...]

  #define DIV_ROUND_CLOSEST_ULL(x, divisor)(		\
  [...]

  #define mult_frac(x, n, d)	\
  ({				\
  [...]

  #define abs_diff(a, b) ({			\
  [...]

I agree your option is the most readable one, hence I used it consistently in
v7.

Thanks,
Cristian

[1] https://lore.kernel.org/all/20260204-rk3588-bgcolor-v7-0-78d1d01c5ca1@collabora.com/