[Intel-gfx] i915 / 3.15 intermittently boots into blank screeen

[Jan Niggemann]

Am 18.07.2014 18:25, schrieb Daniel Vetter:
> On Fri, Jul 18, 2014 at 4:49 PM, Jan Niggemann <jn at hz6.de> wrote:
>>
>> Am 18.07.2014 15:27, schrieb Daniel Vetter:
>>>
>>> On Thu, Jul 17, 2014 at 10:31:30PM +0200, Jan Niggemann wrote:
>>>>
>>>> I'm experiencing an issue with 3.15.5 on my Lenovo T400:
>>>> Since 3.15 (or 3.14, can't say for sure), the boot starts 
>>>> normally, but
>>>> the
>>>> first mode change doesn't occur, resulting in a black screen with
>>>> backlight
>>>> on. The system is entirely unresponsive and I can only press the 
>>>> power
>>>> button until to switch it off.
>>>
>>> I think the only way to move forward here is to double-check that 
>>> 3.14
>>>
>>> works and 3.15 is broken by recompiling with the same .config
>>> (occasionally config changes cause regressions). And then do a full 
>>> git
>>> bisect to find the offending commit.
>>
>> thank you for the feedback.
>> I still have all my custom built kernels, I will test 3.14.0 through 
>> 3.14.8
>> to make sure those were OK and report back.
>
> You only need to test 3.14.0, since the backported fixes only contain
> a very small subset of all patches for 3.15. So it's more efficient 
> to
> then switch to git bisect between 3.14 and 3.15 directly (after 
> you've
> confirmed that 3.15.0 is indeed busted).
I familiarized with git bisect, that was something I had never used 
before.

I started it with "git bisect start v3.15 v3.14 -- 
drivers/gpu/drm/i915"

This lead me to this:

cfa7c862982b431add7f2b362526bf31372fc7b0 is the first bad commit
commit cfa7c862982b431add7f2b362526bf31372fc7b0
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Tue Apr 29 11:53:58 2014 +0200

     drm/i915: Sanitize the enable_ppgtt module option once

     Otherwise we'll end up spamming dmesg on every context creation on 
snb
     with vt-d enabled. This regression was introduced in

     commit 246cbfb5fb9a1ca0997fbb135464c1ff5bb9c549
     Author: Ben Widawsky <benjamin.widawsky at intel.com>
     Date:   Fri Dec 6 14:11:14 2013 -0800

         drm/i915: Reorganize intel_enable_ppgtt

     As the i915.enable_ppgtt is read-only it cannot be changed after 
the
     module is loaded and so we can perform an early sanitization of the
     values.

     v2:
     - Add comment and pimp commit message (Chris)
     - Use the param consistently (Jani)

     v3:
     - Fix init sequence on pre-gen6 by moving the sanitize_ppgtt call 
to
       gtt_init. Fixes boot hangs on pre-gen6.
     - Add a debug output for the sanitize ppgtt mode.

     References: https://lkml.org/lkml/2014/4/17/599
     Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77916
     Cc: Alessandro Suardi <alessandro.suardi at gmail.com>
     Cc: Ben Widawsky <ben at bwidawsk.net>
     Cc: Chris Wilson <chris at chris-wilson.co.uk>
     Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
     Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
     Signed-off-by: Jani Nikula <jani.nikula at intel.com>

:040000 040000 5488e397a1aaa28dca4a252452e9463b0a8f8d10 
214c8e98b3c72844e48ab7aef02cba7daf139fab M	drivers

I realized that the issue does always show, contrary to the subject 
initially chosen.

Unfortunately I can't say anything else, but maybe this will help you 
experts spot the issue.
If I can help more, be it with testing or anything else, just let me 
know.

Regards
jan