drm: add legacy support for using degamma for gamma

The DRM core handles legacy gamma-set ioctl by setting GAMMA_LUT and
clearing CTM and DEGAMMA_LUT.

This works fine on HW where we have either:

degamma -> ctm -> gamma -> out

or

ctm -> gamma -> out

However, if the HW has gamma table before ctm, the atomic property
should be DEGAMMA_LUT, and thus we have:

degamma -> ctm -> out

This is fine for userspace which sets gamma table using the properties,
as the userspace can check for the existence of gamma & degamma, but the
legacy gamma-set ioctl does not work.

Change the DRM core to use DEGAMMA_LUT instead of GAMMA_LUT when the
latter is unavailable.

Signed-off-by: Tomi Valkeinen <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_color_mgmt.c

···
  *
  * There is also support for a legacy gamma table, which is set up by calling
  * drm_mode_crtc_set_gamma_size(). The DRM core will then alias the legacy gamma
- * ramp with "GAMMA_LUT".
+ * ramp with "GAMMA_LUT" or, if that is unavailable, "DEGAMMA_LUT".
  *
  * Support for different non RGB color encodings is controlled through
  * &drm_plane specific COLOR_ENCODING and COLOR_RANGE properties. They
···
 static bool drm_crtc_supports_legacy_gamma(struct drm_crtc *crtc)
 {
 	u32 gamma_id = crtc->dev->mode_config.gamma_lut_property->base.id;
+	u32 degamma_id = crtc->dev->mode_config.degamma_lut_property->base.id;
 
 	if (!crtc->gamma_size)
 		return false;
···
 	if (crtc->funcs->gamma_set)
 		return true;
 
-	return !!drm_mode_obj_find_prop_id(&crtc->base, gamma_id);
+	return !!(drm_mode_obj_find_prop_id(&crtc->base, gamma_id) ||
+		  drm_mode_obj_find_prop_id(&crtc->base, degamma_id));
 }
 
 /**
···
 	struct drm_crtc_state *crtc_state;
 	struct drm_property_blob *blob;
 	struct drm_color_lut *blob_data;
+	u32 gamma_id = dev->mode_config.gamma_lut_property->base.id;
+	u32 degamma_id = dev->mode_config.degamma_lut_property->base.id;
+	bool use_gamma_lut;
 	int i, ret = 0;
 	bool replaced;
 
 	if (crtc->funcs->gamma_set)
 		return crtc->funcs->gamma_set(crtc, red, green, blue, size, ctx);
+
+	if (drm_mode_obj_find_prop_id(&crtc->base, gamma_id))
+		use_gamma_lut = true;
+	else if (drm_mode_obj_find_prop_id(&crtc->base, degamma_id))
+		use_gamma_lut = false;
+	else
+		return -ENODEV;
 
 	state = drm_atomic_state_alloc(crtc->dev);
 	if (!state)
···
 	}
 
 	/* Set GAMMA_LUT and reset DEGAMMA_LUT and CTM */
-	replaced = drm_property_replace_blob(&crtc_state->degamma_lut, NULL);
+	replaced = drm_property_replace_blob(&crtc_state->degamma_lut,
+					     use_gamma_lut ? NULL : blob);
 	replaced |= drm_property_replace_blob(&crtc_state->ctm, NULL);
-	replaced |= drm_property_replace_blob(&crtc_state->gamma_lut, blob);
+	replaced |= drm_property_replace_blob(&crtc_state->gamma_lut,
+					      use_gamma_lut ? blob : NULL);
 	crtc_state->color_mgmt_changed |= replaced;
 
 	ret = drm_atomic_commit(state);

drivers/gpu/drm/drm_fb_helper.c

···
 			goto out_state;
 		}
 
+		/*
+		 * FIXME: This always uses gamma_lut. Some HW have only
+		 * degamma_lut, in which case we should reset gamma_lut and set
+		 * degamma_lut. See drm_crtc_legacy_gamma_set().
+		 */
 		replaced  = drm_property_replace_blob(&crtc_state->degamma_lut,
 						      NULL);
 		replaced |= drm_property_replace_blob(&crtc_state->ctm, NULL);