[Intel-gfx] [PATCH 05/29] drm/i915: prevent NULL pointer exception when using gmbus

Daniel Vetter daniel at ffwll.ch
Sat Apr 14 17:58:52 CEST 2012

On Fri, Apr 13, 2012 at 09:26:14PM -0300, Eugeni Dodonov wrote:
> On Fri, Apr 13, 2012 at 17:18, Chris Wilson <chris at chris-wilson.co.uk>wrote:
> > On Fri, 13 Apr 2012 17:08:41 -0300, Eugeni Dodonov <
> > eugeni.dodonov at intel.com> wrote:
> > > Prevent a NULL pointer exception when we are trying to retrieve EDID data
> > > from non-existent adapter.
> >
> > This just means that a HDMI with a garbage ddc_bus is never detected.
> > Since we control ddc_bus entirely, it means that the initialisation is
> > completely bogus. Please fix the root cause and not paper over
> > programming bugs.
> >
> Sorry, I think I wasn't clear in my cover letter about this one.. I am not
> hitting any issue with this current branch I sent; but the nature of the
> code we have after the gmbus refactoring made me a bit paranoid about
> accessing properties of something that could potential be NULL.
> As far as I can see, we should not hit this with any combination of outputs
> on any hardware that is already covered by the wikipedia :). So I am fine
> with dropping this patch if you also think that it does not adds anything
> relevant to us. Or perhaps I should refactor it into adding some WARN's
> instead?

If you're uneasy about a NULL deref add a BUG_ON (because the NULL-deref
will lead to an oops anyway) or add a WARN_ON and try to bail out. But if
the condition is truely (or at least should be) impossible, a BUG_ON to
document it is imo better (if we can't bail out in a sensible way).
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48

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