[BUG] [i915] black screen on boot -- backlight issue

[Jani Nikula]

On Sat, 03 Feb 2018, Alessandro Caputo <nonedisponibile at gmail.com> wrote:
> I found the following bug:
>
> upon booting, right after the kernel hands over control to init, the
> backlight flashes and then goes to zero. This allows me to see a quick
> flash of getty, but nothing more.

Please file bugs at [1].

> However, everything else is working:
> - I can (blindly) type username and password and successfully log in, start
> x and do things.
> - If I take a screenshot with scrot, then reboot to a working kernel, I can
> indeed see that the screenshot shows my window manager, terminal and
> everything else.
> - If I plug in a HDMI cable to an external display and type the appropriate
> xrandr commands, I can set the external display as primary output and it
> works
>
> Also:
> - I cannot fix the backlight by writing to sysfs
> - if I boot with i915.fastboot=1, everything works (i.e. this is a
> workaround)
>
> I bisected and the first bad commit is the following:
> commit 9301397a63b3bf1090dffe846c6f1c8efa032236

If that's the bad commit, this probably has nothing to do with
backlight. Black screens aren't all about backlight. ;)

Try cherry-picking a979957b66b3 ("drm/i915/edp: Do not do link training
fallback or prune modes on EDP") from drm-tip branch of [2] and see if
it helps. Or try running the drm-tip branch.

>
> Attached you'll find:
> - lspci -vvvnn
> - dmesg and xrandr from a bad bood
> - dmesg and xrandr from a good boot (with i915.fastboot=1)
>
> I am available to test patches or to send you the output of any other
> command.

Please attach the stuff to the bug. Add drm.debug=14 module parameter.


BR,
Jani.


[1] https://bugs.freedesktop.org/enter_bug.cgi?product=DRI&component=DRM/Intel
[2] https://cgit.freedesktop.org/drm/drm-tip


-- 
Jani Nikula, Intel Open Source Technology Center