linux-kernel - Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

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Message-ID: <toxbemskkjxel3xp7ddkon64x4uetiqjw3bnqdxu7kuercxybh@swnbp4dpw2cf>
Date: Mon, 31 Mar 2025 00:04:19 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Christopher Obbard <christopher.obbard@...aro.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        Rui Miguel Silva <rui.silva@...aro.org>,
        Abel Vesa <abel.vesa@...aro.org>
Subject: Re: [PATCH v6] drm/dp: clamp PWM bit count to advertised MIN and MAX
 capabilities

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.silva@...aro.org>
> Signed-off-by: Rui Miguel Silva <rui.silva@...aro.org>
> Signed-off-by: Christopher Obbard <christopher.obbard@...aro.org>

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

-- 
With best wishes
Dmitry