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

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Message-ID: <a9df2561-95d4-47a5-b5df-5874b71937a6@linaro.org>
Date: Thu, 24 Jul 2025 11:42:38 +0200
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Johan Hovold <johan@...nel.org>,
 Christopher Obbard <christopher.obbard@...aro.org>,
 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>,
 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 24/07/2025 11:32, Dmitry Baryshkov wrote:
> On Thu, 24 Jul 2025 at 12:08, <neil.armstrong@...aro.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@...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)); ?
> 
> Per the standard, the min >= 1 and max >= min. We don't need to bother
> about anything here.

Yeah, I agree. But I think a:
if (likely(pn_min <= pn_max))
is simple and doesn't cost much...

> 
> 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

You're right, we need to write it back, but we can't jump to
the tail of the function since it has all the pwmgen logic
in the middle.

Neil

> 
>>
>> 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
>>>
>>
> 
>