Re: [PATCH v7] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

[Dmitry Baryshkov]

On Thu, Aug 14, 2025 at 04:16:09PM +0200, Neil Armstrong wrote:
> From: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> 
> 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.silva@xxxxxxxxxx>
> Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
> Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> Signed-off-by: Neil Armstrong <neil.armstrong@xxxxxxxxxx>
> ---
> Changes in v7:
> - Take in account invalid bit_count
> - Write bit_count back if clamped within BIT_COUNT_CAP_MIN & BIT_COUNT_CAP_MAX 
> - Link to v6: https://lore.kernel.org/all/20250330-wip-obbardc-qcom-t14s-oled-panel-brightness-v6-1-84ad1cd1078a@xxxxxxxxxx 
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>


-- 
With best wishes
Dmitry