Re: [PATCH v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

From: Dmitry Baryshkov
Date: Sun Mar 30 2025 - 13:44:40 EST

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In reply to: Christopher Obbard: "Re: [PATCH v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities"
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On Sun, 30 Mar 2025 at 20:21, Christopher Obbard
<christopher.obbard@xxxxxxxxxx> wrote:
>
> Hi Dmitry,
>
> On Sun, 30 Mar 2025 at 18:17, Dmitry Baryshkov
> <dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Sun, 30 Mar 2025 at 20:11, Christopher Obbard
> > <christopher.obbard@xxxxxxxxxx> wrote:
> > >
> > > Hi Dmitry,
> > >
> > > On Sun, 30 Mar 2025 at 17:42, Dmitry Baryshkov
> > > <dmitry.baryshkov@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> > > > > Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> > > > > ---
> > > > > 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@xxxxxxxxxx
> > > > >
> > > > > 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@xxxxxxxxxx/
> > > > > ---
> > > > > 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.
>
> OK, so just so I understand fully before preparing next version, no
> change is needed in the logic in this line? Just a commit message
> change?

Let me do that for you. This is from your patch:

@@ -4061,21 +4088,6 @@ drm_edp_backlight_probe_max(struct drm_dp_aux
*aux, struct drm_edp_backlight_inf

* - 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) {

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So... I don't know what to say here.

>
>
> > >
> > >
> > > > > + 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.
>
> Sure.
>
> >
> > >
> > >
> > > > > + }
> > > > > + 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.
>
> Sure. I will be more careful in future.
>
>
> > >
> > > >
> > > > > * 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

--
With best wishes
Dmitry

Next message: Dmitry Baryshkov: "Re: [PATCH v4] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities"
Previous message: Dmitry Baryshkov: "Re: [PATCH] drm/bridge:anx7625: Enable DSC feature"
In reply to: Christopher Obbard: "Re: [PATCH v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities"
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