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Message-ID: <CAO9ioeWTgK2AYYt19VSOVjF6rt00xZ=gt1=dUb4A7UEEMwgaSA@mail.gmail.com>
Date: Sun, 30 Mar 2025 20:17:33 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: 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>,
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 v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities
On Sun, 30 Mar 2025 at 20:11, Christopher Obbard
<christopher.obbard@...aro.org> wrote:
>
> Hi Dmitry,
>
> On Sun, 30 Mar 2025 at 17:42, Dmitry Baryshkov
> <dmitry.baryshkov@....qualcomm.com> wrote:
> >
> > On Sun, Mar 30, 2025 at 05:31:20PM +0100, Christopher Obbard 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. 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 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 | 48 ++++++++++++++++++++-------------
> > > 1 file changed, 30 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > > index e2439c8a7fefe116b04aaa689b557e2387b05540..fcc26cb96a51066a503433b2dc660126155d179c 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>
> > > @@ -4033,8 +4034,33 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > > aux->name, ret);
> > > return -ENODEV;
> > > }
> > > -
> >
> > Nitpick: please keep the empty line.
>
> Sure.
>
>
> > > 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) {
> >
> > No. Please take a look a few lines below, where you are removing
> > corresponding lines.
>
> Hmm, the original code which reads CAP_MIN and CAP_MAX both check if
> ret != 1 too, am I missing something ?
Yes, you do. Please scroll this email a few lines down.
>
>
> > > + drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> > > + aux->name, ret);
> > > + return -ENODEV;
> >
> > Hmm. Why? It was 'return 0' before and your commit message contains no
> > explanation.
>
> Yeah, basically returning 0 here would not set bl->max but indicate
> success. Is my logic correct? I will simply update the commit message
> if so.
Please describe that in the commit message why it's required for those
two reg reads to succeed.
>
>
> > > + }
> > > + 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 != 1) {
> > > + 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);
> > > +
> > > bl->max = (1 << pn) - 1;
> > > if (!driver_pwm_freq_hz)
> > > return 0;
> > > @@ -4054,29 +4080,15 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > > */
> > > fxp = DIV_ROUND_CLOSEST(1000 * DP_EDP_BACKLIGHT_FREQ_BASE_KHZ, driver_pwm_freq_hz);
> > >
> > > - /* Use highest possible value of Pn for more granularity of brightness adjustment while
> > > + /*
> > > + * Ensure frequency is within 25% of desired value.
> > > + * Use highest possible value of Pn for more granularity of brightness adjustment while
> >
> > Huh? I don't see a corresponding code change. If you are fixing the
> > comment, it should come as a separate commit.
>
> Sure. I will drop this hunk.
> I folded it into this commit as it was a leftover artifact of moving
> the code around.
Please refrain from folding unrelated changes.
>
> >
> > > * satisfying the conditions below.
> > > * - Pn is in the range of Pn_min and Pn_max
> > > * - F is in the range of 1 and 255
> > > * - FxP is within 25% of desired value.
> > > * Note: 25% is arbitrary value and may need some tweak.
> > > */
> > > - 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 0;
> > > - }
> > > - 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 0;
> > > - }
> > > - pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > - pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > -
> > > - /* Ensure frequency is within 25% of desired value */
> > > fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
> > > fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
> > > if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
> >
> > --
> > With best wishes
> > Dmitry
--
With best wishes
Dmitry
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