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

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Message-Id: <1157969261.23085.108.camel@localhost.localdomain>
Date:	Mon, 11 Sep 2006 11:07:41 +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 19:17 +1000, ysgrifennodd Benjamin Herrenschmidt:
> > >  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.
> 
> In program order. Every time I say "in order", I mean "in program
> order". I agree that this is not enough precision as it's not obvious
> that memcpy will copy in ascending order of addresses (it doesn't have
> to), I'll add that precision... or not. THat could be another question.
> What do we want here ? I would rather have those strongly ordered for
> Class 1.

I'd rather memcpy_to/from_io only made guarantees about the start/end of
the transfer and not order of read/writes or size of read/writes. The
reason being that a more restrictive sequence can be efficiently
expressed using read/writefoo but the reverse is not true.

> > "Except where the underlying device is marked as cachable or
> > prefetchable"
> 
> You aren't supposed to use MMIO accessors on cacheable memory, are you ?

Why not. Providing it is in MMIO space, consider ROMs for example or
write path consider frame buffers.

> with cacheable mappings of anything behind HT... I'd keep use of
> cacheable mapping as an arch specific special case for now, and that
> definitely doesn't allow for MMIO accessors ...

I'm describing existing semantics 8)


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