[Bug 58396] [i945GM regression] in 3.7.0 kernel on macbook1,1 (kernel crash)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=58396

--- Comment #26 from Peter Ujfalusi <peter.ujfalusi at gmail.com> ---
(In reply to comment #25)
> I'll submit it with the bug link updated asap for 3.8, if it passes
> review and all, it should land in a stable backport in 1-2 weeks. But
> holidays might delay that a bit.

Cool, thanks.

> > Out of curiosity: do you have short explanation on why this patch actually
> > helps/works?
> 
> Your BIOS enables VGA on the same pipe 1 as it enables LVDS. When
> setting up the initial kms config we go through all pipes: First we
> disable pipe 0, since it's unused, then we'll enable the desired
> config for LVDS on pipe 1.
> 
> The problem now is that for historical reasons the new i915 modeset
> code disables any outputs when they're disconnected for _any_ modeset,
> i.e. when we ensure that pipe 0 is off, it also wants to disable VGA
> out on pipe 0. Imo this is braindead semantics, but it's bug-for-bug
> compatible with the old modeset implementation from 3.6. The problem
> now is that the new modeset always enables/disables the entire pipe,
> so to disable VGA on pipe it needs to enable/disable LVDS on pipe 1,
> too. There we fall over, since at we don't (yet) read out the mode set
> up by the BIOS, hence the NULL deref oops.

I see. Thank you for taking the time to explain it. It does make perfect sense.
It would have taken a bit of time if I started to read the code and the debug
messages to come to this conclusion (I'm an ASoC guy)...

Thanks again!
Peter

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