diff mbox series

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

Message ID 20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v3-1-156801d97a8a@linaro.org
State New
Headers show
Series [v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities | expand

Commit Message

Christopher Obbard March 30, 2025, 4:31 p.m. UTC 
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@linaro.org>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Christopher Obbard <christopher.obbard@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 | 48 ++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 18 deletions(-)


---
base-commit: 4c4d9b7b6c6e676eca22585139aba5f03de74b90
change-id: 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-4020865b6580

Best regards,

Comments

Dmitry Baryshkov March 30, 2025, 4:42 p.m. UTC | #1 
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@linaro.org>
> Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> Signed-off-by: Christopher Obbard <christopher.obbard@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 | 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.

>  	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.

> +		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.

> +	}
> +	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.

>  	 * 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) {
Dmitry Baryshkov March 30, 2025, 5:17 p.m. UTC | #2 
On Sun, 30 Mar 2025 at 20:11, Christopher Obbard
<christopher.obbard@linaro.org> wrote:
>
> Hi Dmitry,
>
> On Sun, 30 Mar 2025 at 17:42, Dmitry Baryshkov
> <dmitry.baryshkov@oss.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@linaro.org>
> > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> > > Signed-off-by: Christopher Obbard <christopher.obbard@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 | 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
diff mbox series

Patch

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;
 	}
-
 	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) {
+		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 != 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
 	 * 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) {