linux-kernel - Re: [PATCH 3/6] arch/tile: support MMIO-based readb/writeb etc.

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Date:	Mon, 9 Apr 2012 13:24:33 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] arch/tile: support MMIO-based readb/writeb etc.

On Friday 06 April 2012, Chris Metcalf wrote:
> Add support for MMIO read/write on tilegx to support GXIO IORPC access.
> Similar to the asm-generic version, but we include memory fences on
> the writes to be conservative.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@...era.com>

It's usually better to use inline assembly here, to guarantee that
the compiler does not split an access into multiple byte sized
accesses as it might sometimes do if a register data structure
is marged "packed". The "volatile" guarantees that the access
does not go beyond a single word, but it does not guarantee that
it's atomic.

I don't think you need the fences after the write because PCI MMIO
writes are posted anyway (only PIO is non-posted), but you might need
some kind of barrier on the read to prevent a scenario where an MMIO 
read tells you that a DMA has completed, but the CPU (or the compiler)
has scheduled the read of that data ahead of the MMIO read.

	Arnd
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