* Benjamin Herrenschmidt [off-list ref] wrote:

On Wed, 2011-05-18 at 21:16 -0700, Roland Dreier wrote:

quoted

On Wed, May 18, 2011 at 11:31 AM, Milton Miller [off-list ref] wrote:

quoted

So the real question should be why is x86-32 supplying a broken writeq
instead of letting drivers work out what to do it when needed?

Sounds a lot like what I was asking a couple of years ago :)
http://lkml.org/lkml/2009/4/19/164

But Ingo insisted that non-atomic writeq would be fine:
http://lkml.org/lkml/2009/4/19/167

Yuck... Ingo, I think that was very wrong.

Those are for MMIO, which must almost ALWAYS know precisely what the
resulting access size is going to be. It's not even about atomicity
between multiple CPUs. I have seen plenty of HW for which a 64-bit
access to a register is -not- equivalent to two 32-bit ones. In fact, in
some case, you can get the side effects twice ... or none at all.

The only case where you can be lax is when you explicitely know that
there is no side effects -and- the HW cope with different access sizes.
This is not the general case and drivers need at the very least a way to
know what the behaviour will be.

Ok, that's pretty convincing.

Unless hpa or tglx disagrees with reverting this, could any of you send a patch 
with a proper changelog etc. that applies cleanly to v2.6.39?

Thanks,

	Ingo