Re: [PATCH v5] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

[Christopher Obbard]

Hi Dmitry,

On Sun, 30 Mar 2025 at 18:56, Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
>
> On Sun, Mar 30, 2025 at 06:49:40PM +0100, Christopher Obbard wrote:
> > According to the eDP specification (VESA Embedded DisplayPort Standard
> > v1.4b, Section 3.3.10.2), if the value of DP_EDP_PWMGEN_BIT_COUNT is
> > less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, the sink is required to use
> > the MIN value as the effective PWM bit count.
> >
> > This commit updates the logic to clamp the reported
> > DP_EDP_PWMGEN_BIT_COUNT to the range defined by
> > DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN and _CAP_MAX. As part of this change,
> > the behavior is modified such that reading _CAP_MIN and _CAP_MAX
> > registers is now required to succeed. Before reading these registers
> > was optional.
>
> Describe why, not what. Something like 'is now required to succeed,
> otherwise bl->max value can end up being not set, although
> drm_edp_backlight_probe_max() returned success'.
>
> LGTM otherwise.

Amazing. We got there eventually!
I updated the commit message around this change to be:

    As part of this change, the behavior is modified such that reading both
    _CAP_MIN and _CAP_MAX registers is now required to succeed, otherwise
    bl->max value could end up being not set although
    drm_edp_backlight_probe_max() returned success.


I will wait for more feedback for few days before sending the next version.


Cheers!

Chris