drivers/gpu/drm/display/drm_dp_helper.c | 68 ++++++++++++++++++++++++--------- 1 file changed, 49 insertions(+), 19 deletions(-)
From: Christopher Obbard <christopher.obbard@linaro.org>
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 _CAP_MIN and _CAP_MAX.
As part of this change, the behavior is modified such that reading both
_CAP_MIN and _CAP_MAX registers is now required to succeed, otherwise
bl->max value could end up being not set although
drm_edp_backlight_probe_max() returned success.
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>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
---
Changes in v7:
- Take in account invalid bit_count
- Write bit_count back if clamped within BIT_COUNT_CAP_MIN & BIT_COUNT_CAP_MAX
- Link to v6: https://lore.kernel.org/all/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v6-1-84ad1cd1078a@linaro.org
Changes in v6:
- Update commit message around chaning reading PWMGEN_BIT_COUNT_CAP_MIN
and _CAP_MAX to be required.
- Link to v5: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v5-1-25083d9732fc@linaro.org
Changes in v5:
- Correctly check return value when reading PWMGEN_BIT_COUNT_CAP_MIN
and _CAP_MAX.
- Link to v4: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v4-1-85ef0991bdf8@linaro.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 | 68 ++++++++++++++++++++++++---------
1 file changed, 49 insertions(+), 19 deletions(-)
diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index 1ecc3df7e3167d13636e194c4aab44ee8979aa11..011a493f06d4871074832ec954a05697103466b2 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -29,6 +29,7 @@
#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>
@@ -4126,22 +4127,61 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
{
int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
int ret;
- u8 pn, pn_min, pn_max;
+ u8 pn, pn_min, pn_max, bit_count;
if (!bl->aux_set)
return 0;
- ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, &pn);
+ ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, &bit_count);
if (ret < 0) {
drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap: %d\n",
aux->name, ret);
return -ENODEV;
}
- pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+ bit_count &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+ 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 -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 < 0) {
+ 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;
+
+ if (unlikely(pn_min > pn_max)) {
+ drm_dbg_kms(aux->drm_dev, "%s: Invalid pwmgen bit count cap min/max returned: %d %d\n",
+ aux->name, pn_min, pn_max);
+ return -EINVAL;
+ }
+
+ /*
+ * 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.
+ * Only enable this logic if the [MIN, MAX] range is valid in regard to Spec.
+ */
+ pn = bit_count;
+ if (bit_count < pn_min)
+ pn = clamp(bit_count, pn_min, pn_max);
+
bl->max = (1 << pn) - 1;
- if (!driver_pwm_freq_hz)
+ if (!driver_pwm_freq_hz) {
+ if (pn != bit_count)
+ goto bit_count_write_back;
+
return 0;
+ }
/*
* Set PWM Frequency divider to match desired frequency provided by the driver.
@@ -4165,21 +4205,6 @@ 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) {
- 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);
@@ -4197,12 +4222,17 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
break;
}
+bit_count_write_back:
ret = drm_dp_dpcd_write_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, pn);
if (ret < 0) {
drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count: %d\n",
aux->name, ret);
return 0;
}
+
+ if (!driver_pwm_freq_hz)
+ return 0;
+
bl->pwmgen_bit_count = pn;
bl->max = (1 << pn) - 1;
---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250724-topic-x1e80100-t14s-oled-dp-brightness-0190e5201d02
Best regards,
--
Neil Armstrong <neil.armstrong@linaro.org>Hi,
On Thu, 14 Aug 2025 16:16:09 +0200, Neil Armstrong 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 _CAP_MIN and _CAP_MAX.
>
> [...]
Thanks, Applied to https://gitlab.freedesktop.org/drm/misc/kernel.git (drm-misc-next)
[1/1] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities
https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/68a7c52fa9e778bda45ed0b2e83a0bf2ea41b88c
--
Neil
On Thu, Aug 14, 2025 at 04:16:09PM +0200, Neil Armstrong wrote: > From: Christopher Obbard <christopher.obbard@linaro.org> > > 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 _CAP_MIN and _CAP_MAX. > > As part of this change, the behavior is modified such that reading both > _CAP_MIN and _CAP_MAX registers is now required to succeed, otherwise > bl->max value could end up being not set although > drm_edp_backlight_probe_max() returned success. > > 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> > Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org> > --- > Changes in v7: > - Take in account invalid bit_count > - Write bit_count back if clamped within BIT_COUNT_CAP_MIN & BIT_COUNT_CAP_MAX > - Link to v6: https://lore.kernel.org/all/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v6-1-84ad1cd1078a@linaro.org > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com> -- With best wishes Dmitry
Hi Neil,
On Thu, 14 Aug 2025 at 15:16, Neil Armstrong <neil.armstrong@linaro.org> wrote:
>
> From: Christopher Obbard <christopher.obbard@linaro.org>
>
> 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 _CAP_MIN and _CAP_MAX.
>
> As part of this change, the behavior is modified such that reading both
> _CAP_MIN and _CAP_MAX registers is now required to succeed, otherwise
> bl->max value could end up being not set although
> drm_edp_backlight_probe_max() returned success.
>
> 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>
> Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Thank you for taking this patch over. I guess there is no need to add
my tags; but here they are anyway:
Tested-by: Christopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: Christopher Obbard <christopher.obbard@linaro.org>
> ---
> Changes in v7:
> - Take in account invalid bit_count
> - Write bit_count back if clamped within BIT_COUNT_CAP_MIN & BIT_COUNT_CAP_MAX
> - Link to v6: https://lore.kernel.org/all/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v6-1-84ad1cd1078a@linaro.org
>
> Changes in v6:
> - Update commit message around chaning reading PWMGEN_BIT_COUNT_CAP_MIN
> and _CAP_MAX to be required.
> - Link to v5: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v5-1-25083d9732fc@linaro.org
>
> Changes in v5:
> - Correctly check return value when reading PWMGEN_BIT_COUNT_CAP_MIN
> and _CAP_MAX.
> - Link to v4: https://lore.kernel.org/r/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v4-1-85ef0991bdf8@linaro.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 | 68 ++++++++++++++++++++++++---------
> 1 file changed, 49 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index 1ecc3df7e3167d13636e194c4aab44ee8979aa11..011a493f06d4871074832ec954a05697103466b2 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -29,6 +29,7 @@
> #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>
> @@ -4126,22 +4127,61 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> {
> int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> int ret;
> - u8 pn, pn_min, pn_max;
> + u8 pn, pn_min, pn_max, bit_count;
>
> if (!bl->aux_set)
> return 0;
>
> - ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, &pn);
> + ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, &bit_count);
> if (ret < 0) {
> drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap: %d\n",
> aux->name, ret);
> return -ENODEV;
> }
>
> - pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> + bit_count &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> + 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 -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 < 0) {
> + 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;
> +
> + if (unlikely(pn_min > pn_max)) {
> + drm_dbg_kms(aux->drm_dev, "%s: Invalid pwmgen bit count cap min/max returned: %d %d\n",
> + aux->name, pn_min, pn_max);
> + return -EINVAL;
> + }
> +
> + /*
> + * 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.
> + * Only enable this logic if the [MIN, MAX] range is valid in regard to Spec.
> + */
> + pn = bit_count;
> + if (bit_count < pn_min)
> + pn = clamp(bit_count, pn_min, pn_max);
> +
> bl->max = (1 << pn) - 1;
> - if (!driver_pwm_freq_hz)
> + if (!driver_pwm_freq_hz) {
> + if (pn != bit_count)
> + goto bit_count_write_back;
> +
> return 0;
> + }
>
> /*
> * Set PWM Frequency divider to match desired frequency provided by the driver.
> @@ -4165,21 +4205,6 @@ 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) {
> - 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);
> @@ -4197,12 +4222,17 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> break;
> }
>
> +bit_count_write_back:
> ret = drm_dp_dpcd_write_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, pn);
> if (ret < 0) {
> drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count: %d\n",
> aux->name, ret);
> return 0;
> }
> +
> + if (!driver_pwm_freq_hz)
> + return 0;
> +
> bl->pwmgen_bit_count = pn;
> bl->max = (1 << pn) - 1;
>
>
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250724-topic-x1e80100-t14s-oled-dp-brightness-0190e5201d02
>
> Best regards,
> --
> Neil Armstrong <neil.armstrong@linaro.org>
>
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