My knowledge of the DRM is weak, but as I understand it, the only time
it is used is by the ioctl handlers, and not by userspace. I've added
Dave Airlie to the CC list, hopefully he can enlighten us as to where
else cmpxchg is used.
Unless something %100 inside of the kernel will be the only consumers
of this, I'd recommend not adding it. You can't let anything part of
a userland API try to make use of it, and DRM definitely falls into
that category.
istr there was a contentious thread on linux-arch about using cmpxchg in
generic code a while ago. So it's possible architectures with crappy
atomic support will need some kind of hack like this to even be able to
use generic code in the future.
Sparc 32-bit barely has PCI let alone 3D graphics cards.