[Bug 101619] [SKL] DPCD backlight causes black screen on ThinkPad X1 Carbon 4th Gen

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101619

--- Comment #13 from Jenny TC <jenny.tc at intel.com> ---
I faced the same issue and my panel doesnt support AUX Backlight Enable

cat /sys/kernel/debug/dri/0/eDP-1/i915_dpcd 
0000: 12 0a 84 41 00 00 01 c0 02 03 00 00 00 0b 00
0070: 01 00
0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0100: 0a 84 00 00 00 00 00 00 01 00 00
0200: 01 00 77 77 01 01 00 00
0600: 01
0700: 02
0701: bb ff 00 00
0720: 00 00 00 00 06 06 10 00 01 00 00 00 01 80 00 00
0732: 00 14


With below change its working for my panel, but this wont work for the issue
reported in this bug since the panel seems to have wrong DPCD capabilities.

intel_dp_aux_display_control_heuristic(struct intel_connector *connector)
        struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
        uint8_t reg_val;

+       /* Panel dosn't support enabling AUX Backlight control */
+       if (!(intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP)) {
+               return false;
+       }
        /* Panel doesn't support adjusting backlight brightness via PWN pin */
        if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_PWM_PIN_CAP))
                return true;

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