linux-kernel - Re: [PATCH v4] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

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Message-ID: <CACr-zFCN25V0zAttZ_7sSQN3UUEdEdkAv684cuRshqnRtExSKA@mail.gmail.com>
Date: Sun, 30 Mar 2025 18:48:13 +0100
From: Christopher Obbard <christopher.obbard@...aro.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
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>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	linux-arm-msm@...r.kernel.org, Johan Hovold <johan@...nel.org>, 
	Rui Miguel Silva <rui.silva@...aro.org>, Abel Vesa <abel.vesa@...aro.org>
Subject: Re: [PATCH v4] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

On Sun, 30 Mar 2025 at 18:45, Dmitry Baryshkov
<dmitry.baryshkov@....qualcomm.com> wrote:
>
> On Sun, 30 Mar 2025 at 20:31, Christopher Obbard
> <christopher.obbard@...aro.org> wrote:
> >
> > According to the eDP specification (VESA Embedded DisplayPort Standard
> > v1.4b, Section 3.3.10.2), if the value of DP_EDP_PWMGEN_BIT_COUNT is
> > less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, the sink is required to use
> > the MIN value as the effective PWM bit count.
> >
> > This commit updates the logic to clamp the reported
> > DP_EDP_PWMGEN_BIT_COUNT to the range defined by
> > DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN and _CAP_MAX. As part of this change,
> > the behavior is modified such that reading _CAP_MIN and _CAP_MAX
> > registers is now required to succeed. Before reading these registers
> > was optional.
> >
> > This ensures correct handling of eDP panels that report a zero PWM
> > bit count but still provide valid non-zero MIN and MAX capability
> > values. Without this clamping, brightness values may be interpreted
> > incorrectly, leading to a dim or non-functional backlight.
> >
> > For example, the Samsung ATNA40YK20 OLED panel used in the Lenovo
> > ThinkPad T14s Gen6 (Snapdragon) reports a PWM bit count of 0, but
> > supports AUX backlight control and declares a valid 11-bit range.
> > Clamping ensures brightness scaling works as intended on such panels.
> >
> > Co-developed-by: Rui Miguel Silva <rui.silva@...aro.org>
> > Signed-off-by: Rui Miguel Silva <rui.silva@...aro.org>
> > Signed-off-by: Christopher Obbard <christopher.obbard@...aro.org>
> > ---
> > Changes in v4:
> > - Remove unrelated whitespace changes.
> > - Remove unrelated commit change.
> > - Add note to commit message about changing read of PWMGEN_BIT_COUNT_CAP_MIN
> >   and _CAP__MAX from optional to required.
> > - Link to v3: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v3-1-156801d97a8a@linaro.org
> >
> > Changes in v3:
> > - Properly rebase patch on top of latest version of drm-misc-next.
> > - Make patch more generic by clamping PWM bit count to advertised MIN
> >   and MAX capabilities (suggested by Dmitry).
> > - Link to v2: https://lore.kernel.org/r/20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-v2-1-16dc3ee00276@linaro.org
> >
> > Changes in v2:
> > - Split backlight brightness patch from T14s OLED enablement series.
> > - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> > - Rework commit message to reference eDP spec.
> > - Rebase on drm-misc-next.
> > - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
> > ---
> >  drivers/gpu/drm/display/drm_dp_helper.c | 42 +++++++++++++++++++++------------
> >  1 file changed, 27 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > index e2439c8a7fefe116b04aaa689b557e2387b05540..b5c8a75d036b5d65262417de363409e3ff3f4560 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -28,6 +28,7 @@
> >  #include <linux/init.h>
> >  #include <linux/iopoll.h>
> >  #include <linux/kernel.h>
> > +#include <linux/minmax.h>
> >  #include <linux/module.h>
> >  #include <linux/sched.h>
> >  #include <linux/seq_file.h>
> > @@ -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 != 1) {
>
> NAK. See previous discussion.

Sorry, I must be blind. That is blazingly obvious to me now!



> > +               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;
>
>
> --
> With best wishes
> Dmitry