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 <[email protected]> Signed-off-by: Rui Miguel Silva <[email protected]> Signed-off-by: Christopher Obbard <[email protected]> --- 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-v2-1-16dc3ee00...@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-e9bc7c9d3...@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; } - 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) { --- base-commit: 4c4d9b7b6c6e676eca22585139aba5f03de74b90 change-id: 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-4020865b6580 Best regards, -- Christopher Obbard <[email protected]>
