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Message-ID: <CACr-zFCOqGmsFnJ_aW7cV99gmQdEtso3JGEiFFeKjk6-p6R54w@mail.gmail.com>
Date: Sun, 30 Mar 2025 18:11:13 +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 v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities
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 ?
> > + 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.
> > + }
> > + 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.
>
> > * 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
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