[PATCH] drm/amd/display: Scale custom brightness curve from full range

[Akhmed Zhitaev <zhitaevakh@gmail.com>]

Custom brightness curves use an 8-bit input signal.  After exporting the
full PWM range to userspace, the curve normalizer still divides requests
by the physical PWM span.  On panels with a nonzero minimum PWM level,
this can produce a curve input greater than 255 and send an invalid
backlight level to DC.

Scale the userspace [0..max] range to the curve's [0..255] range
instead.  This retains the full advertised range and keeps the reverse
readback conversion unchanged.

Fixes: 8dbd72cb7900 ("drm/amd/display: Export full brightness range to userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Akhmed Zhitaev <zhitaevakh@gmail.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 40d82a3ee..533c69f0b 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -5131,10 +5131,10 @@ static int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
 	return 1;
 }
 
-/* Rescale from [min..max] to [0..AMDGPU_MAX_BL_LEVEL] */
-static inline u32 scale_input_to_fw(int min, int max, u64 input)
+/* Rescale userspace [0..max] to the firmware curve's [0..255]. */
+static inline u32 scale_input_to_fw(int max, u64 input)
 {
-	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max - min);
+	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max);
 }
 
 /* Rescale from [0..AMDGPU_MAX_BL_LEVEL] to [min..max] */
@@ -5147,7 +5147,7 @@ static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *cap
 				      unsigned int min, unsigned int max,
 				      uint32_t *user_brightness)
 {
-	u32 brightness = scale_input_to_fw(min, max, *user_brightness);
+	u32 brightness = scale_input_to_fw(max, *user_brightness);
 	u8 lower_signal, upper_signal, upper_lum, lower_lum, lum;
 	int left, right;
 
-- 
2.55.0