[PATCH] Document Linux's memory barriers [try #4]

[David Howells]

Paul Mackerras <paulus at samba.org> wrote:

> > +On some systems, I/O writes are not strongly ordered across all CPUs, and
> > so +locking should be used, and mmiowb() should be issued prior to
> > unlocking the +critical section.
> 
> I think we should say more strongly that mmiowb() is required where
> MMIO accesses are done under a spinlock, and that if your driver is
> missing them then that is a bug.  I don't think it makes sense to say
> that mmiowb is required "on some systems".

The point I was trying to make was that on some systems writes are not
strongly ordered, so we need mmiowb() on _all_ systems. I'll fix the text to
make that point.

> There shouldn't be any problem here, because readw/writew _must_
> ensure that the device accesses are serialized.

No. That depends on the properties of the memory window readw/writew write
through, the properties of the CPU wrt memory accesses, and what explicit
barriers at interpolated inside readw/writew themselves.

If we're accessing a frame buffer, for instance, we might want it to be able
to reorder and combine reads and writes.

> Of course, on an SMP system it would be quite possible for the
> interrupt to be taken on another CPU, and in that case disabling
> interrupts (I assume that by "DISABLE IRQ" you mean
> local_irq_disable() or some such)

Yes. There are quite a few different ways to disable interrupts.

> gets you absolutely nothing; you need to use a spinlock, and then the mmiowb
> is required.

I believe I've said that, though perhaps not sufficiently clearly.

> You may like to include these words describing some of the rules:

Thanks, I probably will.

David