drm: Fix color LUT rounding

The current implementation of drm_color_lut_extract()
generates weird results. Eg. if we go through all the
values for 16->8bpc conversion we see the following pattern:

in            out (count)
   0 -   7f ->  0 (128)
  80 -  17f ->  1 (256)
 180 -  27f ->  2 (256)
 280 -  37f ->  3 (256)
...
fb80 - fc7f -> fc (256)
fc80 - fd7f -> fd (256)
fd80 - fe7f -> fe (256)
fe80 - ffff -> ff (384)

So less values map to 0 and more values map 0xff, which
doesn't seem particularly great.

To get just the same number of input values to map to
the same output values we'd just need to drop the rounding
entrirely. But perhaps a better idea would be to follow the
OpenGL int<->float conversion rules, in which case we get
the following results:

in            out (count)
   0 -   80 ->  0 (129)
  81 -  181 ->  1 (257)
 182 -  282 ->  2 (257)
 283 -  383 ->  3 (257)
...
fc7c - fd7c -> fc (257)
fd7d - fe7d -> fd (257)
fe7e - ff7e -> fe (257)
ff7f - ffff -> ff (129)

Note that since the divisor is constant the compiler
is able to optimize away the integer division in most
cases. The only exception is the _ULL() case on 32bit
architectures since that gets emitted as inline asm
via do_div() and thus the compiler doesn't get to
optimize it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231013131402.24072-2-ville.syrjala@linux.intel.comReviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>

include/drm/drm_color_mgmt.h

@@ -36,20 +36,17 @@ struct drm_plane;
  *
  * Extract a degamma/gamma LUT value provided by user (in the form of
  * &drm_color_lut entries) and round it to the precision supported by the
- * hardware.
+ * hardware, following OpenGL int<->float conversion rules
+ * (see eg. OpenGL 4.6 specification - 2.3.5 Fixed-Point Data Conversions).
  */
 static inline u32 drm_color_lut_extract(u32 user_input, int bit_precision)
 {
-	u32 val = user_input;
-	u32 max = 0xffff >> (16 - bit_precision);
-
-	/* Round only if we're not using full precision. */
-	if (bit_precision < 16) {
-		val += 1UL << (16 - bit_precision - 1);
-		val >>= 16 - bit_precision;
-	}
-
-	return clamp_val(val, 0, max);
+	if (bit_precision > 16)
+		return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(user_input, (1 << bit_precision) - 1),
+					     (1 << 16) - 1);
+	else
+		return DIV_ROUND_CLOSEST(user_input * ((1 << bit_precision) - 1),
+					 (1 << 16) - 1);
 }
 
 u64 drm_color_ctm_s31_32_to_qm_n(u64 user_input, u32 m, u32 n);