On Fri, 28 Mar 2025 at 11:25, Dmitry Baryshkov <dmitry.barysh...@oss.qualcomm.com> wrote: > > On Thu, 27 Mar 2025 at 20:19, Christopher Obbard > <christopher.obb...@linaro.org> wrote: > > > > Hi Dmitry, > > > > On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov > > <dmitry.barysh...@oss.qualcomm.com> wrote: > > > > > > On 27/03/2025 19:25, Christopher Obbard wrote: > > > > According to the eDP specification (e.g., VESA eDP 1.4b, section > > > > 3.3.10.2), > > > > if 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 bit count. > > > > > > > > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still > > > > providing > > > > valid non-zero MIN and MAX capability values. This patch updates the > > > > logic > > > > to use the CAP_MIN value in such cases, ensuring correct scaling of > > > > AUX-set > > > > backlight brightness values. > > > > > > > > This improves compatibility with panels like the Samsung ATNA40YK20 used > > > > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a > > > > bit count of 0 but declares an 11-bit PWM capability range. > > > > > > > > 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> > > > > --- > > > > 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 | 50 > > > > ++++++++++++++++++++++----------- > > > > 1 file changed, 33 insertions(+), 17 deletions(-) > > > > > > > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c > > > > b/drivers/gpu/drm/display/drm_dp_helper.c > > > > index > > > > dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 > > > > 100644 > > > > --- a/drivers/gpu/drm/display/drm_dp_helper.c > > > > +++ b/drivers/gpu/drm/display/drm_dp_helper.c > > > > @@ -4083,7 +4083,7 @@ 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, bl_caps; > > > > > > > > if (!bl->aux_set) > > > > return 0; > > > > @@ -4094,8 +4094,39 @@ 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_readb(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 0; > > > > + } > > > > + pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK; > > > > + > > > > + ret = drm_dp_dpcd_readb(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 0; > > > > + } > > > > + pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK; > > > > + > > > > + ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, > > > > &bl_caps); > > > > + if (ret != 1) { > > > > + bl_caps = 0; > > > > + drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight > > > > adjustment cap: %d\n", > > > > + aux->name, ret); > > > > + } > > > > + > > > > + /* > > > > + * Some eDP panels report brightness byte count support, but the > > > > byte count > > > > + * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead. > > > > + */ > > > > + if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT) > > > > + && pn_min) > > > > + pn = pn_min; > > > > > > I wonder, what stops you from implementing this part according to the > > > standard, rather than adding a hack for 0 value. > > > > I am simply quite dense, I do not know this spec or the DRM code well > > at all ;-). > > Okay, no worries. It should be pretty easy: > > pn = clamp(pn_min, pn_max); > > No need to check for pn being non-zero, etc.
I think you probably mean: pn = clamp(pn, pn_min, pn_max); I will look to use this macro in the next version later today & make it more generic; thanks ! PS: I also found someone who tried to fix this in 2019; I will add the author to CC of the next version. https://lore.kernel.org/r/dri-devel/a766c3498d3754b598a8bf66f59a76e78ec57080.ca...@intel.com/T/
