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

[Ахмед Житаев]

Yes, this is still needed.

f1b5d8f9cc54 fixes the advertised max_brightness mismatch by restoring
the full userspace range [0..max].

The issue fixed by this patch is different: the custom brightness curve
input is still normalized by (max - min). On panels with a non-zero PWM
minimum, valid userspace requests near the top of the [0..max] range can
therefore produce a curve input above 255.

On my panel, min=3084 and max=65535. Without this patch, requests above
the upper boundary caused the backlight to drop dark; with this patch,
the full range maps into [0..255] and the issue no longer reproduces.

I also tested with the f1b5d8f9cc54 semantics present, so the regression
is still present without this change.


чт, 13 авг. 2026 г. в 22:45, Mario Limonciello <mario.limonciello@xxxxxxx>:
>
>
>
> 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: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Akhmed Zhitaev <zhitaevakh@xxxxxxxxx>
> > ---
> >   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;
> >
>
> Is this still needed with
>
> "drm/amd/display: Fix backlight max_brightness to match exported range"
>
> https://github.com/torvalds/linux/commit/f1b5d8f9cc54ae8a2567ac126867ae488e1bf625