linux-kernel - Re: ARM unaligned MMIO access with attribute((packed))

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Date:	Wed, 02 Feb 2011 13:38:31 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	linux@....linux.org.uk
Cc:	arnd@...db.de, linux-arm-kernel@...ts.infradead.org,
	linux-usb@...r.kernel.org, Ulrich.Weigand@...ibm.com,
	gcc@....gnu.org, linux-kernel@...r.kernel.org,
	peter.maydell@...aro.org
Subject: Re: ARM unaligned MMIO access with attribute((packed))

From: Russell King - ARM Linux <linux@....linux.org.uk>
Date: Wed, 2 Feb 2011 16:37:02 +0000

> 1. there's no way to tell GCC that the inline assembly is a load
>    instruction and therefore it needs to schedule the following
>    instructions appropriately.

Just add a dummy '"m" (pointer)' asm input argument to the inline asm
statement.  Just make sure "typeof(pointer)" has a size matching the
size of the load your are performing.

> 2. GCC will needlessly reload pointers from structures and other such
>    behaviour because it can't be told clearly what the inline assembly
>    is doing, so the inline asm needs to have a "memory" clobber.

This behavior is correct, and in fact needed.  Writing to chip registers
can trigger changes to arbitrary main memory locations.

> 3. It seems to misses out using the pre-index addressing, prefering to
>    create add/sub instructions prior to each inline assembly load/store.

Yes, this is indeed a problem.

But you really need that memory clobber there whether you like it or
not, see above.
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