[Bug 29787] [RADEON:KMS::EDID] i2c bit banging + preempt kernel -> i2c failure (random XRandR failures)

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https://bugs.freedesktop.org/show_bug.cgi?id=29787

--- Comment #18 from Ville Syrjala <syrjala@sci.fi> 2011-05-21 04:31:35 PDT ---
(In reply to comment #15)
> (In reply to comment #14)
> > Grumble. So, how will this be fixed? Sprinkle the code with
> > preempt_disable()/enable() pairs? Advise all users (well, me) to disable
> > preemption? Change the code so it no longer acts on intermittent EDID failures?
> > Ask your colleagues to design sane hardware that doesn't rely on software
> > interaction for timing-critical operations ;) ?
> > 
> > FWIW, I'm perfectly happy to disable preemption in my kernel, haven't run a
> > distro-provided kernel since 2.6.27. But your explanation sounds like this
> > could bite a lot more users than just me.
> 
> It has nothing to do with the hw, it's the nature of i2c.  All drivers that use
> bit banging i2c (hwmon, v4l, lots of embedded stuff) would potentially be
> affected.

If the EEPROM supports clock stretching I see no reason why bit banging would
fail.

I got these timeout errors on one system at some point. I had an idea for a
possible fix, but I never got around to testing it. I'll attach the patch.

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