Re: [rfc PATCH] ieee1394: ohci1394: delete bogus spinlock, flush MMIO writes

On Wed, 29 Nov 2006 00:50:43 +0100
Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx> wrote:

Alan wrote:
On Tue, 28 Nov 2006 22:24:11 +0100 (CET)
Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx> wrote:
All MMIO writes which were surrounded by the spinlock as well as the
very last MMIO write of the IRQ handler are now explicitly flushed by
MMIO reads of the respective register.

MMIO is ordered anyway on the bus, you just need mmiowb() to force
ordering to the bus controller in case you are on a big numa box.

The mmiowb is a checkpoint to ensure ordering between different threads
of MMIO writes; i.e. it doesn't halt the thread until the write actually
reached the device like a read would do, right?

It guarantees that no other mmio will sneak past it from another thread
but doesn't guarantee the previous I/O has hit the hardware. It's a much
weaker (and thus far faster) guarantee which is usually sufficient as it
can be combined with spin_unlock to enforce I/O ordering matching the
lock ordering.

Alan
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