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

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Message-ID: <6612cd14-8353-4d3a-a248-5d32e0d3ca23@linaro.org>
Date: Thu, 24 Jul 2025 11:08:43 +0200
From: neil.armstrong@...aro.org
To: Johan Hovold <johan@...nel.org>,
 Christopher Obbard <christopher.obbard@...aro.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, Rui Miguel Silva <rui.silva@...aro.org>,
 Abel Vesa <abel.vesa@...aro.org>
Subject: Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX
 capabilities

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@...nel.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@...nel.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)); ?

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
>