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Message-ID: <aJC0GLAeGneb3WFR@hovoldconsulting.com>
Date: Mon, 4 Aug 2025 15:22:32 +0200
From: Johan Hovold <johan@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
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 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.
(But there is no need for likely() as this is not a hot path.)
Johan
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