x86/iommu: Don't make AMD_GART depend on EXPERT and default y

x86/iommu: Don't make AMD_GART depend on EXPERT and default y

The AMD_GART driver was made EXPERT/EMBEDDED a long time
ago to avoid unbootable 64bit systems with 32bit only devices.

This was before swiotlb was there, which does the job
of this fallback today. SWIOTLB is always on, so systems
should always boot.

The drawback is that every system has to compile that
driver in (it cannot be a module).

Also:
 - Newer AMD CPUs (the APUs) don't seem to have AMD_GART support
   at all anymore.

 - Newer AMD platforms have a much better real IOMMU

 - The AMD GART driver was never very good (lots of overhead,
   e.g. in flushing due to some workarounds) and it's doubtful it's
   really better than SWIOTLB.

 - On older K8 systems it didn't even work with all chipsets.

 - The 32bit device bounce buffer case should be rare/
   non performance critical these days anyways.

 - On non AMD systems it is not needed at all.

So drop the EXPERT dependency on AMD_GART and remove the
default y. The driver can be still compiled in, just
it's an explicit decision now, and people who don't want
it can unselect it.

I also clarified the description a bit.

This allows to save ~8K text on most modern x86-64 systems.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1380922676-23007-1-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>