drm: Add color management LUT validation helper (v4)

Some hardware may place additional restrictions on the gamma/degamma
curves described by our LUT properties.  E.g., that a gamma curve never
decreases or that the red/green/blue channels of a LUT's entries must be
equal.  Let's add a helper function that drivers can use to test that a
userspace-provided LUT is valid and doesn't violate hardware
requirements.

v2:
 - Combine into a single helper that just takes a bitmask of the tests
   to apply.  (Brian Starkey)
 - Add additional check (always performed) that LUT property blob size
   is always a multiple of the LUT entry size.  (stolen from ARM driver)

v3:
 - Drop the LUT size check again since
   drm_atomic_replace_property_blob_from_id() already covers this for
   us.  (Alexandru Gheorghe)

v4:
 - Use an enum to describe possible test values rather than #define's;
   this is cleaner to provide kerneldoc for.  (Daniel Vetter)
 - s/DRM_COLOR_LUT_INCREASING/DRM_COLOR_LUT_NON_DECREASING/.  (Ville)

Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Swati Sharma <swati2.sharma@intel.com>
Cc: Brian Starkey <Brian.Starkey@arm.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217224415.12848-1-matthew.d.roper@intel.com

drivers/gpu/drm/drm_color_mgmt.c

@@ -462,3 +462,47 @@ int drm_plane_create_color_properties(struct drm_plane *plane,
 	return 0;
 }
 EXPORT_SYMBOL(drm_plane_create_color_properties);
+
+/**
+ * drm_color_lut_check - check validity of lookup table
+ * @lut: property blob containing LUT to check
+ * @tests: bitmask of tests to run
+ *
+ * Helper to check whether a userspace-provided lookup table is valid and
+ * satisfies hardware requirements.  Drivers pass a bitmask indicating which of
+ * the tests in &drm_color_lut_tests should be performed.
+ *
+ * Returns 0 on success, -EINVAL on failure.
+ */
+int drm_color_lut_check(struct drm_property_blob *lut,
+			uint32_t tests)
+{
+	struct drm_color_lut *entry;
+	int i;
+
+	if (!lut || !tests)
+		return 0;
+
+	entry = lut->data;
+	for (i = 0; i < drm_color_lut_size(lut); i++) {
+		if (tests & DRM_COLOR_LUT_EQUAL_CHANNELS) {
+			if (entry[i].red != entry[i].blue ||
+			    entry[i].red != entry[i].green) {
+				DRM_DEBUG_KMS("All LUT entries must have equal r/g/b\n");
+				return -EINVAL;
+			}
+		}
+
+		if (i > 0 && tests & DRM_COLOR_LUT_NON_DECREASING) {
+			if (entry[i].red < entry[i - 1].red ||
+			    entry[i].green < entry[i - 1].green ||
+			    entry[i].blue < entry[i - 1].blue) {
+				DRM_DEBUG_KMS("LUT entries must never decrease.\n");
+				return -EINVAL;
+			}
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_color_lut_check);

include/drm/drm_color_mgmt.h

@@ -69,4 +69,33 @@ int drm_plane_create_color_properties(struct drm_plane *plane,
 				      u32 supported_ranges,
 				      enum drm_color_encoding default_encoding,
 				      enum drm_color_range default_range);
+
+/**
+ * enum drm_color_lut_tests - hw-specific LUT tests to perform
+ *
+ * The drm_color_lut_check() function takes a bitmask of the values here to
+ * determine which tests to apply to a userspace-provided LUT.
+ */
+enum drm_color_lut_tests {
+	/**
+	 * @DRM_COLOR_LUT_EQUAL_CHANNELS:
+	 *
+	 * Checks whether the entries of a LUT all have equal values for the
+	 * red, green, and blue channels.  Intended for hardware that only
+	 * accepts a single value per LUT entry and assumes that value applies
+	 * to all three color components.
+	 */
+	DRM_COLOR_LUT_EQUAL_CHANNELS = BIT(0),
+
+	/**
+	 * @DRM_COLOR_LUT_NON_DECREASING:
+	 *
+	 * Checks whether the entries of a LUT are always flat or increasing
+	 * (never decreasing).
+	 */
+	DRM_COLOR_LUT_NON_DECREASING = BIT(1),
+};
+
+int drm_color_lut_check(struct drm_property_blob *lut,
+			uint32_t tests);
 #endif