Messages in this thread | | | Date | Wed, 02 Feb 2011 13:38:31 -0800 (PST) | Subject | Re: ARM unaligned MMIO access with attribute((packed)) | From | David Miller <> |
| |
From: Russell King - ARM Linux <linux@arm.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.
| |