linux-kernel - Re: MMIO and gcc re-ordering issue

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Date:	Wed, 11 Jun 2008 14:18:41 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <matthew@....cx>,
	Trent Piepho <tpiepho@...escale.com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, linuxppc-dev@...abs.org,
	alan@...rguk.ukuu.org.uk, linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

Nick Piggin writes:

> OK, I'm sitll not quite sure where this has ended up. I guess you are happy
> with x86 semantics as they are now. That is, all IO accesses are strongly
> ordered WRT one another and WRT cacheable memory (which includes keeping
> them within spinlocks),

My understanding was that on x86, loads could pass stores in general,
i.e. a later load could be performed before an earlier store.  I guess
that can't be true for uncached loads, but could a cacheable load be
performed before an earlier uncached store?

> - as strong as x86. guaranteed not to break drivers that work on x86,
>   but slower on some archs. To me, this is most pleasing. It is much
>   much easier to notice something is going a little slower and to work
>   out how to use weaker ordering there, than it is to debug some
>   once-in-a-bluemoon breakage caused by just the right architecture,
>   driver, etc. It totally frees up the driver writer from thinking
>   about barriers, provided they get the locking right.

I just wish we had even one actual example of things going wrong with
the current rules we have on powerpc to motivate changing to this
model.

> Now that doesn't leave waker ordering architectures lumped with "slow old
> x86 semantics". Think of it as giving them the benefit of sharing x86
> development and testing :) We can then formalise the relaxed __ accessors
> to be more complete (ie. +/- byteswapping).

That leaves a gulf between the extremely strongly ordered writel
etc. and the extremely weakly ordered __writel etc.  The current
powerpc scheme is fine for a lot of drivers but your proposal would
leave us no way to deliver it to them.

Paul.
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