linux-kernel - Re: [PATCH 1/9] ARM i.MX51: Add ipu clock support

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Date:	Wed, 15 Dec 2010 17:49:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Russell King - ARM Linux" <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Sascha Hauer <s.hauer@...gutronix.de>,
	"Zhang Lily-R58066" <r58066@...escale.com>,
	linux-fbdev@...r.kernel.org,
	Arnaud Patard <arnaud.patard@...-net.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] ARM i.MX51: Add ipu clock support

On Wednesday 15 December 2010, Russell King - ARM Linux wrote:
> > The regular accessor function for I/O registers is readl, which handles
> > the access correctly with regard to atomicity, I/O ordering and byteorder.
> 
> There's no possibility of those two being mis-ordered - they will be in
> program order whatever.
> 
> What isn't guaranteed is the ordering between I/O accesses (accesses to
> device memory) and SDRAM accesses (normal memory) which can pass each other
> without additional barriers.  Memory accesses can pass I/O accesses.

Yes, that's what I meant.

> If you don't need normal vs device access ordering, using readl_relaxed()/
> writel_relaxed() is preferred, and avoids the (apparantly rather high)
> performance overhead of having to issue barriers all the way down to the
> L2 cache.

Well, my point was that the authors should choose their I/O accessors
carefully. Using __raw_writel() without any explanations is a rather
bad default, it's not designed for that. Using writel() as a default
is usually a good choice, as we can assume it to do the right thing.

writel_relaxed() is also good where appropriate, because it tells
the reader that the driver author has thought about the I/O (vs. code)
ordering and concluded that it's safe to do.

> Lastly, I don't see where atomicity comes into it - __raw_writel vs writel
> have the same atomicity.  Both are single access atomic provided they're
> naturally aligned.  Misaligned device accesses are not predictable.

This is just what gcc turns it into today. In theory, a future gcc or
a future cpu might change that. If you mark a pointer as
'__attribute__((packed))', it probably already does, even for aligned
pointers, while it does not when using writel_{,relaxed}. The point is
that __raw_* means just that -- we don't give any guarantees on what
happens on the bus, so people should not use it.

	Arnd
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