Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

On Thu, 24 Jul 2025 at 12:08, <neil.armstrong@linaro.org> wrote:
>
> On 20/05/2025 10:06, Johan Hovold wrote:
> > Hi Chris,
> >
> > On Fri, Apr 04, 2025 at 02:24:32PM +0100, Christopher Obbard wrote:
> >> On Fri, 4 Apr 2025 at 09:54, Johan Hovold <johan@kernel.org> wrote:
> >>> On Fri, Apr 04, 2025 at 08:54:29AM +0100, Christopher Obbard wrote:
> >>>> On Mon, 31 Mar 2025 at 09:33, Johan Hovold <johan@kernel.org> wrote:
> >>>>>> @@ -4035,6 +4036,32 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> >>>>>>        }
> >>>>>>
> >>>>>>        pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> >>>>>> +
> >>>>>> +     ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> >>>>>> +     if (ret < 0) {
> >>>>>> +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> >>>>>> +                         aux->name, ret);
> >>>>>> +             return -ENODEV;
> >>>>>> +     }
> >>>>>> +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> >>>>>> +
> >>>>>> +     ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> >>>>>> +     if (ret < 0) {
> >>>>>> +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> >>>>>> +                         aux->name, ret);
> >>>>>> +             return -ENODEV;
> >>>>>> +     }
> >>>>>> +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> >>>>>> +
> >>>>>> +     /*
> >>>>>> +      * Per VESA eDP Spec v1.4b, section 3.3.10.2:
> >>>>>> +      * If DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> >>>>>> +      * the sink must use the MIN value as the effective PWM bit count.
> >>>>>> +      * Clamp the reported value to the [MIN, MAX] capability range to ensure
> >>>>>> +      * correct brightness scaling on compliant eDP panels.
> >>>>>> +      */
> >>>>>> +     pn = clamp(pn, pn_min, pn_max);
> >>>>>
> >>>>> You never make sure that pn_min <= pn_max so you could end up with
> >>>>> pn < pn_min on broken hardware here. Not sure if it's something you need
> >>>>> to worry about at this point.
>
> I'm trying to figure out what would be the behavior in this case ?
>
> - Warn ?
> - pn_max = pn_min ?
> - use BIT_COUNT as-is and ignore MIN/MAX ?
> - pm_max = max(pn_min, pn_max); pm_min = min(pn_min, pn_max); ?
> - reverse clamp? clamp(pn, pn_max, pn_min); ?
> - generic clamp? clamp(pn, min(pn_min, pn_max), max(pn_min, pn_max)); ?

Per the standard, the min >= 1 and max >= min. We don't need to bother
about anything here.

On the other hand, I think the patch needs to be updated a bit. If the
pn value changed after clamping, it makes sense to write it back to
the DP_EDP_PWMGEN_BIT_COUNT register by jumping to the tail of the
drm_edp_backlight_probe_max() function

>
> Or just bail out ?
>
> Neil
>
> >>>>
> >>>> I am honestly not sure. I would hope that devices follow the spec and
> >>>> there is no need to be too paranoid, but then again we do live in the
> >>>> real world where things are... not so simple ;-).
> >>>> I will wait for further feedback from someone who has more experience
> >>>> with eDP panels than I have.
> >>>
> >>> There's always going to be buggy devices and input should always be
> >>> sanitised so I suggest adding that check before calling clamp() (which
> >>> expects min <= max) so that the result here is well-defined.
> >>
> >> Makes sense, I will do so in the next revision.
> >
> > It seems you never got around to respinning this one so sending a
> > reminder.
> >
> > Johan
> >
>


-- 
With best wishes
Dmitry