tree 12c85eebd88793ddf7cb0643063ff93c8755dfae
parent 0bfe151cc4049f3f304adf28b37ea5437d02ad96
author Roland Dreier <rolandd@cisco.com> 1209502013 -0700
committer Roland Dreier <rolandd@cisco.com> 1209502013 -0700

IB/mthca: Avoid changing userspace ABI to handle DMA write barrier attribute

Commit cb9fbc5c ("IB: expand ib_umem_get() prototype") changed the
mthca userspace ABI to provide a way for userspace to indicate which
memory regions need the DMA write barrier attribute.  However, it is
possible to handle this without breaking existing userspace, by having
the mthca kernel driver recognize whether it is talking to old or new
userspace, depending on the size of the register MR structure passed in.

The only potential drawback of this is that is allows old userspace
(which has a bug with DMA ordering on large SGI Altix systems) to
continue to run on new kernels, but the advantage of allowing old
userspace to continue to work on unaffected systems seems to outweigh
this, and we can print a warning to push people to upgrade their
userspace.

Signed-off-by: Roland Dreier <rolandd@cisco.com>