Messages in this thread | | | Date | Wed, 29 Nov 2006 01:26:56 +0000 | From | Alan <> | Subject | Re: [rfc PATCH] ieee1394: ohci1394: delete bogus spinlock, flush MMIO writes |
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On Wed, 29 Nov 2006 00:50:43 +0100 Stefan Richter <stefanr@s5r6.in-berlin.de> wrote:
> Alan wrote: > > On Tue, 28 Nov 2006 22:24:11 +0100 (CET) > > Stefan Richter <stefanr@s5r6.in-berlin.de> 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.
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