[Bug 75791] New: i915: backlight turns off during boot

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=75791

            Bug ID: 75791
           Summary: i915: backlight turns off during boot
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.14.3
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: mcy at lm7.fr
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

Created attachment 135521
  --> https://bugzilla.kernel.org/attachment.cgi?id=135521&action=edit
Output of dmesg

Hello,

Since I upgraded from Linux 3.13 (Debian package) to Linux 3.14 (Debian
package 3.14.2-1, confirmed by compiling a 3.14.3 from kernel.org with
the same options), the backlight of my laptop (MacBook4,1) turns
completely off when booting, right after “Waiting for /dev to be fully
populated” is printed on the console. If I wait for the computer to
fully boot, I can log in to KDE (via KDM), and then the brightness is
set to my default level, and can be adjusted with the dedicated keys on
the keyboard (Fn + F1/F2 in my case).

I'm guessing the problem is simply that the brightness is set to the
minimum, but on this machine the minimum possible brightness disables
the backlight completely.

For information:
$ cat /sys/class/backlight/intel_backlight/max_brightness
494

It's a bit tricky to get the minimal visible brightness, because setting
various values in /sys/class/backlight/intel_backlight/brightness
doesn't always give the same result, but it seems that a good minimum
would be 99 (20% in KDE). Unfortunately, I can't seem to find a kernel
parameter that would allow to set the base brightness.

I did found two work-arounds:

1. If I pass i915.invert_brightness=1 to the kernel, brightness stays at
   the maximum during the boot time, but (obviously) the brightness
   level is reversed (so I have to push the “brightness +” button to
   lower the brightness, KDE sets a higher brightness when idle, etc.),
   so that's not ideal.

2. If I pass nomodeset or i915.modeset=0 to the kernel, brightness stays
   at the maximum but can't be adjusted; /sys/class/backlight is empty.

I also tried a bunch of other parameters without noticeable effect
(video.use_native_backlight=1, acpi_backlight=vendor, etc.).

Re-compiling the kernel with CONFIG_DRM_I915_UMS=y and
CONFIG_DRM_I915_KMS disabled to force user mode setting instead of
kernel mode setting makes the display work at boot-time, but
when launching KDM it starts being all glitchy, and it stays buggy when
switching to a console. The brightness is set at the maximum and can't
be adjusted, and /sys/class/backlight is empty.

Not sure this is relevant, but as you can guess from the machine type,
it's an EFI boot.

I'm attaching the output of dmesg right after booting the computer, and
the output of lspci -vvv. I'm at your service if you need any other
information or testing.

Cheers,
  Matteo

-- 
You are receiving this mail because:
You are on the CC list for the bug.
You are the assignee for the bug.