Date:	Fri, 10 Aug 2012 18:05:47 +0200
From:	Mihai Moldovan <ionic@...ic.de>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: null pointer dereference while loading i915

* On 10.08.2012 12:10 PM, Daniel Vetter wrote:
> On Wed, Aug 8, 2012 at 6:50 AM, Mihai Moldovan <ionic@...ic.de> wrote:
>> Hi Daniel, hi list
>>
>> ever since version 3.2.0 (maybe even earlier, but 3.0.2 is still working fine),
>> my box is crashing when loading the i915 driver (mode-setting enabled.)
>>
>> The current version I'm testing with is 3.5.0.
>>
>> I was able to get the BUG output (please forgive any errors/flips in the output,
>> I have had to transcribe the messages from the screen/images), however, I'm not
>> able to find out what's wrong.
>>
>> If I see it correctly, there's a null pointer dereference in a printk called
>> from inside gmbus_xfer. The only printk calls I can see in
>> drivers/gpu/drm/i915/intel_i2c.c gmbus_xfer() however are issued by the
>> DRM_DEBUG_KMS() and DRM_INFO() macros.
>> Neither call looks wrong to me, I even tried to swap adapter->name with
>> bus->adapter.name and make *sure* i < num is true, but haven't had any success.
>>
>> I'd really like to see this bug fixed, as it's preventing me from updating the
>> kernel for over a year now.
>>
>> Also, while 3.0.2 works, it *does* spew error/warning messages related to gmbus
>> and I've had corrupted VTs in the past (albeit after a long uptime with multiple
>> X restarting and DVI cable unplugging/reattaching events), so maybe there's a
>> lot more broken than "expected".
>
> Hm, this is rather strange. gmbus should not be enable on 3.2 nor 3.0,
> since exactly this issue might happen. We've re-enabled gmbus again on
> 3.5 after having fixed this bug. Are you sure that this is plain 3.2
> you're running?

Sorry, I messed up the version numbers. Started bisecting yesterday and noticed,
that 3.0 up to 3.2 still work "fine" (see below), instead I've had another
problem with 3.2 (completely lockup after the kernel is running for a few
minutes, but I have no idea where this issue is coming from. Seems to be
happening with 3.2.0 only, so... *shrug*)

3.0.2           => working, gmbus warnings as posted.
3.1-09933/07170 => working, NO gmbus warnings, but render errors (see below)
3.2-rc2 to rc4  => working, NO gmbus warnings, but render errors (see below)
--- (stopped bisecting 3.0 to 3.2 as this was pointless) ---
--- (restarted bisecting with 3.2 to 3.5) ---
3.3.0-06109     => working, gmbus warnings just like with 3.0, render errors
(see below)
3.4.0-07487     => working, gmbus warnings, hang errors (see below)
...

I've done more steps, but have not yet finished bisecting, so stay tuned.
All those render errors look like that:

[drm] capturing error event; look for more information in
/debug/dri/0/i915_error_state
render error detected, EIR: 0x00000010
  IPEIR: 0x00000000
  IPEHR: 0x02000000
  INSTDONE: 0xffffffff
  INSTPS: 0x8001e025
  INSTDONE1: 0xbfbbffff
  ACTHD: 0x00a4203c
page table error
  PGTBL_ER: 0x00100000
[drm:i915_report_and_clear_eir] *ERROR* EIR stuck: 0x00000010, masking

I'll finish bisecting (and hope, that my guess was right, concerning the
varaiant I wasn't able to build) and will post the bisect log when done.

Meanwhile: at least for 3.0.2 and even older versions, gmbus must have been
enabled as I'm pretty sure I always saw those errors when booting (just
confirmed via logs for 3.0.0, 26.38.6, 2.6.39). Doesn't come up with 2.6.34,
2.6.36.1, 3.1-..., 3.2-... though.

Best regards,


Mihai



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