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

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Message-Id: <e0d58a$78jc@orsmga002.jf.intel.com>
Date:	Wed, 18 May 2011 09:04:11 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Keith Packard <keithp@...thp.com>, Mihai Moldovan <ionic@...ic.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915/sdvo: Reorder i2c initialisation before ddc proxy

On Tue, 17 May 2011 18:22:52 -0700, Keith Packard <keithp@...thp.com> wrote:
> On Tue, 17 May 2011 14:03:50 +0100, Chris Wilson <chris@...is-wilson.co.uk> 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

-- 
Chris Wilson, Intel Open Source Technology Centre
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