Re: [PATCH] drm/i915/sdvo: Reorder i2c initialisation before ddc proxy

From: Chris Wilson
Date: Wed May 18 2011 - 04:06:25 EST

On Tue, 17 May 2011 18:22:52 -0700, Keith Packard <keithp@xxxxxxxxxx> wrote:
> On Tue, 17 May 2011 14:03:50 +0100, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> > The ddc proxy depends upon the underlying i2c bus being selected. Under
> > certain configurations, the i2c-adapter functionality is queried during
> > initialisation and so may trigger an OOPS during boot. Hence, we need to
> > reorder the initialisation of the ddc proxy until after we hook up the i2c
> > adapter for the SDVO device.
> I'd love more explanation here about how this code ever worked -- what
> are these 'certain configurations' of which you speak?

The condition under which it fails is when the i2c_add_adapter calls into
i2c_detect which will attempt to probe all valid addresses on the adapter
iff there is a pre-existing i2c_driver with the same class as the freshly
added i2c_adapter.

So it appears to depend upon having compiled in (or loaded such a module
before i915.ko) an i2c-driver that likes to futz over the i2c_adapters
claiming DDC support.

Chris Wilson, Intel Open Source Technology Centre
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