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.
+
bl->max = (1 << pn) - 1;
if (!driver_pwm_freq_hz)
return 0;
--
With best wishes
Dmitry
