[Bug 108773] i915.fastboot=1 is broken on Thinkpad S1 Yoga (Haswell)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108773

--- Comment #19 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to Johannes Krampf from comment #18)
> Created attachment 144329 [details]
> Dmesg of drm-tip with fastboot on T440s
> 
> This is drm-tip version 307cedb7b907ee533bf9e746d0e06bea04f2722b from Thu
> May 23 12:49:55 2019 +1000. I used drm.debug=0xe on the kernel flags as
> requested. Previously I had 0x1e.
> 
> The behaviour changed slightly with drm-tip:
> 
> On startup, I still get the sub-second flash with boot messages and then the
> screen _including the backlight_ turns off. I can interact with the system
> (wayland+VT), but get no screen output.

OK. I guess we're dealing with a backlight problem at this point.

We do see the telltale signs here:
[    0.910355] [drm:lpt_setup_backlight] CPU backlight register was enabled,
switching to PCH override

The code was supposed to be fixed so that it can do the CPU->PCH switch but it
looks like it's still not working correctly.

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