On 1/29/26 11:37 AM, Andy Shevchenko wrote:

On Thu, Jan 29, 2026 at 02:58:52AM +0200, Cristian Ciocaltea wrote:

quoted

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.

...

quoted

+/*
+ * 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. :-(

quoted

+ */
+#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/ (local)