[lkml]   [2011]   [Feb]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: ARM unaligned MMIO access with attribute((packed))
From: Russell King - ARM Linux <>
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.

 \ /
  Last update: 2011-02-02 22:41    [W:0.187 / U:32.692 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site