edp backtrace spam on MacBookAir4,1

[Daniel Vetter]

On Wed, May 30, 2012 at 08:39:13AM -0700, Linus Torvalds wrote:
> On Wed, May 30, 2012 at 1:27 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> >
> > Ok, Chris couldn't reproduce this on his mba. Can you please boot with
> > drm.debug=0xe, reproduce the noise and then attach the full dmesg?
> 
> Hmm. Now *I* can't reproduce it either.
> 
> I have updated my system in the meantime, so maybe this is related to
> that. However, I suspect it's more likely that it's some race
> condition, because when I got it, I got a *lot* of it, but they were
> all very tightly bunched together:
> 
>  [ 1588.996413] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1588.996650] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.000983] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.001225] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.005975] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.006218] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.010980] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.011224] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.015976] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.016211] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.020986] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
>  [ 1589.021232] WARNING: at drivers/gpu/drm/i915/intel_dp.c:350
> intel_dp_check_edp+0x5d/0xb0()
> 
> ie that's 12 of those warnings (each of them with that huge backtrace
> etc), but they are all within 0.03 seconds of each other. So I suspect
> it needs to hit some particular timing window, and I was just
> (un)lucky.
> 
> Because when I try it now with DRM debugging, I can't hit it. And just
> to make sure, I re-did the test without debugging too (in case the
> debugging would have changed timing), but can't reproduce it that way
> either.

Hm, that's pretty strange that you can't reproduce this any more. We check
this has_edp stuff once at boot and then never touch it again. Getting all
these backtraces in close bunches is expected, because we have a lot of
these checks - because we've had a lot of bugs :(. Did you by chance
upgrade the firmware or switch from efi booting to bios booting - we use
the vbios to detect part of the edp configuration?

The only other thing I could think of is whether disabling the internal
panel changes anything. Iirc that's an edp panel on macbook airs, so this
could paper over most of the warnings if it's enabled.
-Daniel
-- 
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48