[Intel-gfx] [PATCH 1/2 v2 addendum] drm/i915: Allow parsing of variable size child device entries from VBT

[David Weinehall]

On Wed, Aug 12, 2015 at 03:13:35PM +0300, Jani Nikula wrote:
> On Wed, 12 Aug 2015, David Weinehall <david.weinehall at linux.intel.com> wrote:
> > Some more fixup is needed; the bits from Antti's patch
> > that actually expanded the struct to fully fit the newer
> > versions of the child_device_config was part of the second
> > patch; since that patch hasn't been merged yet we need this bit:
> >
> > This applies on top of the patch you already merged
> > (the Iboost patch will need corresponding adjustment to
> >  remove the changes I split out):
> >
> > Expand common_child_dev_config to be able to fit all information
> > defined by the latest VBT specification.
> >
> > Signed-off-by: David Weinehall <david.weinehall at linux.intel.com>
> > CC: Antti Koskipaa <antti.koskipaa at linux.intel.com>
> > ---
> >  intel_bios.c |    7 ++++++-
> >  intel_bios.h |    4 ++++
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_bios.c b/drivers/gpu/drm/i915/intel_bios.c
> > index 990acc20771a..40e2cc4e7419 100644
> > --- a/drivers/gpu/drm/i915/intel_bios.c
> > +++ b/drivers/gpu/drm/i915/intel_bios.c
> > @@ -1038,6 +1038,10 @@ parse_device_mapping(struct drm_i915_private *dev_priv,
> >  		DRM_DEBUG_KMS("No general definition block is found, no devices defined.\n");
> >  		return;
> >  	}
> > +	/* Remember to keep this in sync with child_device_config;
> > +	 * whenever a new feature is added to BDB that causes that
> > +	 * struct to grow this needs to be updated too
> > +	 */
> 
> BUILD_BUG_ON(something about sizeof child device config) ?

The idea is nice, but...  We get the size to copy (expected_size)
from the version-switch statement (so it's not available during build);
thus we cannot know at compile time whether expected_size is larger than
sizeof(child_device_config).

This is yet another argument in favour of a version:feature
table I think; that would allow for compile-time validation.

Unless someone else volunteers to refactor this code I can dig in once
I get back from vacation.


Kind regards, David