On Sun, Mar 30, 2025 at 08:31:07PM +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 _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.si...@linaro.org> > Signed-off-by: Rui Miguel Silva <rui.si...@linaro.org> > Signed-off-by: Christopher Obbard <christopher.obb...@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.barysh...@oss.qualcomm.com> -- With best wishes Dmitry
