Kernel 4.7 (re)introduces the problem that looks like bug 76276 for me. I have a Dell XPS13 that works correctly under 4.6, showing an acpi_video0 device under /sys/class/backlight that is of type "firmware". When booting 4.7 this is replaced by intel_backlight of type "raw" and the backlight slowly (aprox. 1 Hz) pulsates. Passing video.use_bios_initial_backlight=0 kernel parameter does not help.
Neither does video.use_bios_initial_backlight=1 btw.
Just tried video.use_native_backlight=0 but that doesn't help either. There is still no acpi_video0 device, only intel_backlight.
The bugreference is to freedesktop bugtracker, my apologies https://bugs.freedesktop.org/show_bug.cgi?id=76276
I finally managed to fix the problem by reintroducing the quickhack mentioned in comment #15 https://bugs.freedesktop.org/show_bug.cgi?id=76276#c15 i.e. hard-disable intel_panel_setup_backlight in intel_panel.c
(In reply to Martin from comment #0) > Kernel 4.7 (re)introduces the problem that looks like bug 76276 for me. I > have a Dell XPS13 that works correctly under 4.6, showing an acpi_video0 > device under /sys/class/backlight that is of type "firmware". > > When booting 4.7 this is replaced by intel_backlight of type "raw" and the > backlight slowly (aprox. 1 Hz) pulsates. Passing > video.use_bios_initial_backlight=0 kernel parameter does not help. That cmdline option isn't used to decide which backlight interface to use. I don't see XPS13 in the video's quirk table so I don't think ACPI plays any magic with this model. Did you use any cmdline option in the working v4.6 kernel? If not, I don't see why intel_backlight didn't show up in v4.6.
The only commandline I use in 4.6 and 4.7 is i915.enable_rc6=7. I reported the problem on freedestkop here: https://bugs.freedesktop.org/show_bug.cgi?id=97295 Funny to see how freedesktop i915 developer deflects accusations as well ;)
Funny how I'm here too. Please either try v4.8-rc1 or do a git bisect to find the regressing commit.
That remark was in jest, it was not my intention to offend you! I just tested 4.8-rc1 and it exhibits the exact same problem. Will start bisect ASAP, don't expect miracles.
I get stuck in 4.6.0-rc3+ land which leaves i915 in unusable state (boots to black screen, no way of telling what backlight driver won). I skipped many revisions and even tried to skip 4.6.0-rc3 completely by using "git skip v4.6.0-rc3 v4.6.0-rc4" but even that resulted in a 4.6.0-rc3+ kernel (how?) that was unusable. Any tips on how to circumvent v4.6.0-rc3?
BTW, since you mentioned bug 76276, I went there but got: Bug #76276 does not exist Typo perhaps?
(In reply to Martin from comment #3) > The bugreference is to freedesktop bugtracker, my apologies > https://bugs.freedesktop.org/show_bug.cgi?id=76276 Oh I see.
It looks like adding drm.debug=0xe to kernel cmdline on the broken v4.7 kernel may give us some hint. Please do that and attach dmesg, thanks.
Created attachment 228661 [details] dmesg of 4.7 booting with intel_backlight device dmesg drm debug output of kernel 4.7 booting with faulty intel_backlight device.
Forgot to say: also need the dmesg of the working v4.6 to compare. The drm.debug=0xe is still need with v4.6. Thanks.
Created attachment 228691 [details] dmesg output 4.6.6 with acpi_video0 drm debug dmesg output of 4.6.6 with functional acpi_video0 device
OK, that clears the mystery why intel_backlight is back. dmesg_4.6.6_acpi_video0.log:[ 0.643470] [drm:parse_lfp_backlight] PWM backlight not present in VBT (type 0) dmesg_4.7.0_intel_backlight.log:[ 0.765087] [drm:parse_lfp_backlight] VBT backlight PWM modulation frequency 10000 Hz, active high, min brightness 0, level 255 Jani, Any ideas?
Created attachment 230861 [details] fix backlight regression on xps 13 l332x I run in the same regression since v4.7, this seems to have fixed it.
Although the patch does not apply cleanly on 4.7.2 (dev_priv was renamed dev recently?), making the exception for IVB in intel_opregion_get_panel_type as well fixes the problem. Result is a correctly working acpi_video0 backlight device, just like the quickhack.
Please follow this bug at the freedesktop bugzilla page as it's not related to ACPI as comment #16 showed.