Re: [PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero

[Dmitry Baryshkov]

On Thu, 27 Mar 2025 at 20:19, Christopher Obbard
<christopher.obbard@xxxxxxxxxx> wrote:
>
> Hi Dmitry,
>
> On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov
> <dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
> >
> > On 27/03/2025 19:25, Christopher Obbard wrote:
> > > According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> > > if 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 bit count.
> > >
> > > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> > > valid non-zero MIN and MAX capability values. This patch updates the logic
> > > to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> > > backlight brightness values.
> > >
> > > This improves compatibility with panels like the Samsung ATNA40YK20 used
> > > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> > > bit count of 0 but declares an 11-bit PWM capability range.
> > >
> > > Co-developed-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> > > Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> > > Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> > > ---
> > > Changes in v2:
> > > - Split backlight brightness patch from T14s OLED enablement series.
> > > - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> > > - Rework commit message to reference eDP spec.
> > > - Rebase on drm-misc-next.
> > > - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@xxxxxxxxxx/
> > > ---
> > >   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
> > >   1 file changed, 33 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > > index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> > > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > > @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > >   {
> > >       int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> > >       int ret;
> > > -     u8 pn, pn_min, pn_max;
> > > +     u8 pn, pn_min, pn_max, bl_caps;
> > >
> > >       if (!bl->aux_set)
> > >               return 0;
> > > @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > >                           aux->name, ret);
> > >               return -ENODEV;
> > >       }
> > > -
> > >       pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> > > +     if (ret != 1) {
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> > > +                         aux->name, ret);
> > > +             return 0;
> > > +     }
> > > +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> > > +     if (ret != 1) {
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> > > +                         aux->name, ret);
> > > +             return 0;
> > > +     }
> > > +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> > > +     if (ret != 1) {
> > > +             bl_caps = 0;
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> > > +                     aux->name, ret);
> > > +     }
> > > +
> > > +     /*
> > > +      * Some eDP panels report brightness byte count support, but the byte count
> > > +      * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> > > +      */
> > > +     if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > > +         && pn_min)
> > > +             pn = pn_min;
> >
> > I wonder, what stops you from implementing this part according to the
> > standard, rather than adding a hack for 0 value.
>
> I am simply quite dense, I do not know this spec or the DRM code well
> at all ;-).

Okay, no worries. It should be pretty easy:

pn = clamp(pn_min, pn_max);

No need to check for pn being non-zero, etc.

> I do appreciate your continued reviews though, however painful it must be.

It's not, no worries.

-- 
With best wishes
Dmitry