drm/i915/color: Downscale degamma lut values read from hardware

For MTL and beyond, convert back the 24 bit lut values
read from HW to 16 bit values to maintain parity with
userspace values. This way we avoid pipe config mismatch
for pre-csc lut values.

v2: Add helper function to downscale values (Jani)

Signed-off-by: Chaitanya Kumar Borah <[email protected]>
Reviewed-by: Uma Shankar <[email protected]>
Signed-off-by: Ankit Nautiyal <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: ckborah

drivers/gpu/drm/i915/display/intel_color.c

@@ -3456,6 +3456,14 @@ static struct drm_property_blob *glk_read_degamma_lut(struct intel_crtc *crtc)
 	for (i = 0; i < lut_size; i++) {
 		u32 val = intel_de_read_fw(dev_priv, PRE_CSC_GAMC_DATA(pipe));
 
+		/*
+		 * For MTL and beyond, convert back the 24 bit lut values
+		 * read from HW to 16 bit values to maintain parity with
+		 * userspace values
+		 */
+		if (DISPLAY_VER(dev_priv) >= 14)
+			val = change_lut_val_precision(val, 16, 24);
+
 		lut[i].red = val;
 		lut[i].green = val;
 		lut[i].blue = val;