drm: Add CRTC background color property

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.

Co-developed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Diederik de Haas <diederik@cknow-tech.com>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://patch.msgid.link/20260303-rk3588-bgcolor-v8-2-fee377037ad1@collabora.com
Signed-off-by: Daniel Stone <daniels@collabora.com>

4 files changed, 100 insertions, 1 deletions

diff --git a/include/drm/drm_blend.h b/include/drm/drm_blend.h
index 88bdfec3bd88..c7e888767c81 100644
--- a/include/drm/drm_blend.h
+++ b/include/drm/drm_blend.h
@@ -31,8 +31,9 @@
 #define DRM_MODE_BLEND_COVERAGE		1
 #define DRM_MODE_BLEND_PIXEL_NONE	2
 
-struct drm_device;
 struct drm_atomic_state;
+struct drm_crtc;
+struct drm_device;
 struct drm_plane;
 
 static inline bool drm_rotation_90_or_270(unsigned int rotation)
@@ -58,4 +59,5 @@ int drm_atomic_normalize_zpos(struct drm_device *dev,
 			      struct drm_atomic_state *state);
 int drm_plane_create_blend_mode_property(struct drm_plane *plane,
 					 unsigned int supported_modes);
+void drm_crtc_attach_background_color_property(struct drm_crtc *crtc);
 #endif
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 66278ffeebd6..312fc1e745d2 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -275,6 +275,18 @@ struct drm_crtc_state {
 	struct drm_property_blob *gamma_lut;
 
 	/**
+	 * @background_color:
+	 *
+	 * RGB value representing the CRTC's background color.  The background
+	 * color (aka "canvas color") of a CRTC is the color that will be used
+	 * for pixels not covered by a plane, or covered by transparent pixels
+	 * of a plane.  The value here should be built using DRM_ARGB64_PREP*()
+	 * helpers, while the individual color components can be extracted with
+	 * desired precision via the DRM_ARGB64_GET*() macros.
+	 */
+	u64 background_color;
+
+	/**
 	 * @target_vblank:
 	 *
 	 * Target vertical blank period when a page flip
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 5e1dd0cfccde..687c0ee163d2 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -836,6 +836,11 @@ struct drm_mode_config {
 	 * gamma LUT as supported by the driver (read-only).
 	 */
 	struct drm_property *gamma_lut_size_property;
+	/**
+	 * @background_color_property: Optional CRTC property to set the
+	 * background color.
+	 */
+	struct drm_property *background_color_property;
 
 	/**
 	 * @suggested_x_property: Optional connector property with a hint for
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 3693d82b5279..a4bdc4bd11bc 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -27,6 +27,9 @@
 #ifndef _DRM_MODE_H
 #define _DRM_MODE_H
 
+#include <linux/bits.h>
+#include <linux/const.h>
+
 #include "drm.h"
 
 #if defined(__cplusplus)
@@ -1549,6 +1552,83 @@ struct drm_mode_closefb {
 	__u32 pad;
 };
 
+/*
+ * 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.
+ * Also note bpc must be greater than 0.
+ */
+#define __DRM_ARGB64_PREP(c, shift)					\
+	(((__u64)(c) & __GENMASK(15, 0)) << (shift))
+
+#define __DRM_ARGB64_PREP_BPC(c, shift, bpc)				\
+({									\
+	__u16 mask = __GENMASK((bpc) - 1, 0);				\
+	__u16 conv = __KERNEL_DIV_ROUND_CLOSEST((mask & (c)) *		\
+						__GENMASK(15, 0), mask);\
+	__DRM_ARGB64_PREP(conv, shift);					\
+})
+
+#define DRM_ARGB64_PREP(alpha, red, green, blue)			\
+(									\
+	__DRM_ARGB64_PREP(alpha, 48) |					\
+	__DRM_ARGB64_PREP(red,   32) |					\
+	__DRM_ARGB64_PREP(green, 16) |					\
+	__DRM_ARGB64_PREP(blue,   0)					\
+)
+
+#define DRM_ARGB64_PREP_BPC(alpha, red, green, blue, bpc)		\
+({									\
+	__typeof__(bpc) __bpc = bpc;					\
+	__DRM_ARGB64_PREP_BPC(alpha, 48, __bpc) |			\
+	__DRM_ARGB64_PREP_BPC(red,   32, __bpc) |			\
+	__DRM_ARGB64_PREP_BPC(green, 16, __bpc) |			\
+	__DRM_ARGB64_PREP_BPC(blue,   0, __bpc);			\
+})
+
+/*
+ * Extract the specified color component from a standard 64-bit ARGB value.
+ *
+ * If the requested precision is less than 16 bits, make use of a conversion
+ * ratio calculated as (2^bpc - 1) / (2^16 - 1), where bpc and 16 are the
+ * output and input precision, respectively.
+ *
+ * If speed is more important than accuracy, use DRM_ARGB64_GET*_BPCS()
+ * instead of DRM_ARGB64_GET*_BPC() in order to replace the expensive
+ * division with a simple bit right-shift operation.
+ */
+#define __DRM_ARGB64_GET(c, shift)					\
+	((__u16)(((__u64)(c) >> (shift)) & __GENMASK(15, 0)))
+
+#define __DRM_ARGB64_GET_BPC(c, shift, bpc)				\
+({									\
+	__u16 comp = __DRM_ARGB64_GET(c, shift);			\
+	__KERNEL_DIV_ROUND_CLOSEST(comp * __GENMASK((bpc) - 1, 0),	\
+				   __GENMASK(15, 0));			\
+})
+
+#define __DRM_ARGB64_GET_BPCS(c, shift, bpc)				\
+	(__DRM_ARGB64_GET(c, shift) >> (16 - (bpc)))
+
+#define DRM_ARGB64_GETA(c)		__DRM_ARGB64_GET(c, 48)
+#define DRM_ARGB64_GETR(c)		__DRM_ARGB64_GET(c, 32)
+#define DRM_ARGB64_GETG(c)		__DRM_ARGB64_GET(c, 16)
+#define DRM_ARGB64_GETB(c)		__DRM_ARGB64_GET(c, 0)
+
+#define DRM_ARGB64_GETA_BPC(c, bpc)	__DRM_ARGB64_GET_BPC(c, 48, bpc)
+#define DRM_ARGB64_GETR_BPC(c, bpc)	__DRM_ARGB64_GET_BPC(c, 32, bpc)
+#define DRM_ARGB64_GETG_BPC(c, bpc)	__DRM_ARGB64_GET_BPC(c, 16, bpc)
+#define DRM_ARGB64_GETB_BPC(c, bpc)	__DRM_ARGB64_GET_BPC(c, 0, bpc)
+
+#define DRM_ARGB64_GETA_BPCS(c, bpc)	__DRM_ARGB64_GET_BPCS(c, 48, bpc)
+#define DRM_ARGB64_GETR_BPCS(c, bpc)	__DRM_ARGB64_GET_BPCS(c, 32, bpc)
+#define DRM_ARGB64_GETG_BPCS(c, bpc)	__DRM_ARGB64_GET_BPCS(c, 16, bpc)
+#define DRM_ARGB64_GETB_BPCS(c, bpc)	__DRM_ARGB64_GET_BPCS(c, 0, bpc)
+
 #if defined(__cplusplus)
 }
 #endif