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

[Mario Limonciello]

On 8/13/26 12:09, Akhmed Zhitaev wrote:

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: sta...@vger.kernel.org
Signed-off-by: Akhmed Zhitaev <zhitaev...@gmail.com>

Thanks for this.

We have moved the code in amd-staging-drm-next

https://gitlab.freedesktop.org/agd5f/linux/-/commit/6c2b46e9774f13e45985f6da9f7d5f8458767b98

As your changes are trivial, I'll adjust while commiting.

Reviewed-by: Mario Limonciello (AMD) <supe...@kernel.org>

---
  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;