linux-kernel - Re: [RFC] MMIO accessors & barriers documentation

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Date:	Mon, 11 Sep 2006 09:57:51 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jeff Garzik <jgarzik@...ox.com>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [RFC] MMIO accessors & barriers documentation

Ar Llu, 2006-09-11 am 14:03 +1000, ysgrifennodd Benjamin Herrenschmidt:
> be interleaved when reaching the host PCI controller (and thus the

"a host PCI controller". The semantics with multiple independant PCI
busses are otherwise evil.

>  1- {read,write}{b,w,l,q} : Those accessors provide all MMIO ordering
> requirements. They are thus called "fully ordered". That is #1, #2 and
> #4 for writes and #1 and #3 for reads. 

#4 may be incredibly expensive on NUMA boxes.

>  3- memcpy_to_io, memcpy_from_io: #1 semantics apply (all MMIO loads or
> stores are performed in order to each other). #2+#4 (stores) or #3

What is "in order" here. "In ascending order of address" would be
tighter.

>  1- __{read,write}{b,w,l,q} : Those accessors provide only ordering rule
> #1. That is, MMIOs are ordered vs. each other as issued by one CPU.
> Barriers are required to ensure ordering vs. memory and vs. locks (see
> "Barriers" section). 

"Except where the underlying device is marked as cachable or
prefetchable"

Q2:
> coherency domain. If we decide not to, then an explicit barrier will
> still be needed in most drivers before spin_unlock(). This is the
> current mmiowb() barrier that I'm proposing to rename (section * III *).

I think we need mmiowb() still anyway (for __writel etc)

> If we decide to not enforce rule #4 for ordered accessors, and thus
> require the barrier before spin_unlock, the above trick, could still be
> implemented as a debug option to "detect" the lack of appropriate
> barriers.

This I think is an excellent idea.

> [* Question 3] If we decide that accessors of Class 1 do not provide rule
> #4, then this barrier is to be used for all classes of accessors, except
> maybe PIO which should always be fully ordered.

On x86 PIO (outb/inb) etc are always ordered and always stall until the
cycle completes on the device.

> [* Question 5] Should we document the rules for memory-memory barriers
> here as well ? (and give examples, like live updating of a network
> driver ring descriptor entry)
> 

Update the existing docs


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