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

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Message-ID: <kg52pqpr5ygfw3usru4z5e6vniqiexiewyplpru433tilvwyzi@stwgmyxie4k7>
Date: Tue, 5 Aug 2025 08:07:19 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Johan Hovold <johan@...nel.org>
Cc: Neil Armstrong <neil.armstrong@...aro.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 Mon, Aug 04, 2025 at 03:22:32PM +0200, Johan Hovold wrote:
> On Thu, Jul 24, 2025 at 02:09:10PM +0300, Dmitry Baryshkov wrote:
> > On 24/07/2025 12:42, Neil Armstrong wrote:
> > > 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:
> > >>>> 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..
> > 
> > Really, no need to.
> 
> It doesn't matter what the spec says, what matters is what may happen if
> a device violates the spec (e.g. if a driver triggers a division by
> zero).
> 
> Always sanitise your input.

Agreed. I hope Chris will now post v7...

> 
> (But there is no need for likely() as this is not a hot path.)
> 
> Johan

-- 
With best wishes
Dmitry