On Sun, 30 Mar 2025 at 20:31, Christopher Obbard <[email protected]> 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 > DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN and _CAP_MAX. As part of this change, > the behavior is modified such that reading _CAP_MIN and _CAP_MAX > registers is now required to succeed. Before reading these registers > was optional. > > 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 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-v3-1-156801d97...@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-v2-1-16dc3ee00...@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 | 42 > +++++++++++++++++++++------------ > 1 file changed, 27 insertions(+), 15 deletions(-) > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c > b/drivers/gpu/drm/display/drm_dp_helper.c > index > e2439c8a7fefe116b04aaa689b557e2387b05540..b5c8a75d036b5d65262417de363409e3ff3f4560 > 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> > @@ -4035,6 +4036,32 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, > struct drm_edp_backlight_inf > } > > 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) {
NAK. See previous discussion. > + 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; -- With best wishes Dmitry
