[ofa-general] Re: [PATCH] for-2.6.23 ib/umad: add partition support

[Roland Dreier]

 > Ugh. OFED 1.2 (with the old ABI) just went out.
 > I wonder - is it time to start making the kernel backwards-compatible?
 > It would be trivial to have userspace supply its own ABI
 > version and have kernel support both new and old ABI if we want to.
 > What do you think?

There's always a balance between keeping cruft in the kernel for
compatibility and not breaking userspace.  I'm beginning to think the
right plan in this case might be to rename struct ib_user_mad_hdr to
struct ib_user_mad_hdr_old, make a new struct ib_user_mad with the
pkey_index member and add a new ioctl IB_USER_MAD_ENABLE_PKEY_INDEX.

The ABI version would stay the same, and if someone just opened the
device and didn't do the IB_USER_MAD_ENABLE_PKEY_INDEX they would get
the old ABI.  If they do the ioctl then they get the new header.  Also
we could define that ABI version 6 just has the new struct
ib_user_mad_hdr and no ioctl.

Then we could say we were going to switch to the new ABI in a year or
two.  And print a warning in the kernel log for every application that
doesn't use the ioctl.

I'll try to cook up a kernel patch next week.

 - R.