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: Rui Miguel Silva <[e-mail]>
Signed-off-by: Rui Miguel Silva <[e-mail]>
Signed-off-by: Christopher Obbard <[e-mail]>
Tested-by: Christopher Obbard <[e-mail]>
Reviewed-by: Christopher Obbard <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/2[e-mail]

drivers/gpu/drm/display/drm_dp_helper.c

···
 #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>
···
 {
 	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.
···
 	 * - 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);
···
 			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;