Commit 68a7c52f authored by Christopher Obbard's avatar Christopher Obbard Committed by Neil Armstrong
Browse files

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



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: default avatarRui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: default avatarRui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: default avatarChristopher Obbard <christopher.obbard@linaro.org>
Tested-by: default avatarChristopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: default avatarChristopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: default avatarDmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: default avatarNeil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250814-topic-x1e80100-t14s-oled-dp-brightness-v7-1-b3d7b4dfe8c5@linaro.org
parent 36842189

drivers/gpu/drm/display/drm_dp_helper.c

+49 −19
Original line number Diff line number Diff line
@@ -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>
@@ -4136,22 +4137,61 @@ 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.
@@ -4175,21 +4215,6 @@ 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);
@@ -4207,12 +4232,17 @@ 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;