drm/vkms: Support setting custom background color

Commit 0b9eff726d56 upstream

Make use of the BACKGROUND_COLOR CRTC property when filling the
background during blending.  It already defaults to solid black.

Since the internal representation of the pixel color in VKMS relies on
16 bits of precision, use the newly introduced DRM_ARGB64_GET{R|G|B}()
helpers to access the individual components of the background color
property, which is compliant with DRM_FORMAT_ARGB16161616.

It's worth noting the alpha component is ignored, hence non-opaque
background colors are not supported.

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Diederik de Haas <diederik@cknow-tech.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://patch.msgid.link/20260303-rk3588-bgcolor-v8-3-fee377037ad1@collabora.com
Signed-off-by: Daniel Stone <daniels@collabora.com>

Author: cristicc

drivers/gpu/drm/vkms/vkms_composer.c

@@ -372,8 +372,14 @@ static void blend(struct vkms_writeback_job *wb,
 {
 	struct vkms_plane_state **plane = crtc_state->active_planes;
 	u32 n_active_planes = crtc_state->num_active_planes;
-
-	const struct pixel_argb_u16 background_color = { .a = 0xffff };
+	u64 bgcolor = crtc_state->base.background_color;
+
+	const struct pixel_argb_u16 background_color = {
+		.a = 0xffff,
+		.r = DRM_ARGB64_GETR(bgcolor),
+		.g = DRM_ARGB64_GETG(bgcolor),
+		.b = DRM_ARGB64_GETB(bgcolor),
+	};
 
 	int crtc_y_limit = crtc_state->base.mode.vdisplay;
 	int crtc_x_limit = crtc_state->base.mode.hdisplay;

drivers/gpu/drm/vkms/vkms_crtc.c

@@ -4,6 +4,7 @@
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_blend.h>
 #include <drm/drm_managed.h>
 #include <drm/drm_probe_helper.h>
 #include <drm/drm_vblank.h>
@@ -296,6 +297,8 @@ struct vkms_output *vkms_crtc_init(struct drm_device *dev, struct drm_plane *pri
 
 	drm_crtc_enable_color_mgmt(crtc, 0, false, VKMS_LUT_SIZE);
 
+	drm_crtc_attach_background_color_property(crtc);
+
 	spin_lock_init(&vkms_out->lock);
 	spin_lock_init(&vkms_out->composer_lock);