[PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores

On Thu, 25 Oct 2012, Rob Herring wrote:

On 10/25/2012 09:16 AM, Johannes Stezenbach wrote:

quoted

On Thu, Oct 25, 2012 at 07:41:45AM -0500, Rob Herring wrote:

quoted

On 10/25/2012 04:34 AM, Johannes Stezenbach wrote:

quoted

On Thu, Oct 11, 2012 at 07:43:22AM -0500, Rob Herring wrote:

quoted

While v6 can support unaligned accesses, it is optional and current
compilers won't emit unaligned accesses. So we don't clear the A bit for
v6.

not true according to the gcc changes page

What are you going to believe: documentation or what the compiler
emitted? At least for ubuntu/linaro 4.6.3 which has the unaligned access
support backported and 4.7.2, unaligned accesses are emitted for v7
only. I guess default here means it is the default unless you change the
default in your build of gcc.

Since ARMv6 can handle unaligned access in the same way as ARMv7
it seems a clear bug in gcc which might hopefully get fixed.
Thus in this case I think it is reasonable to follow the
gcc documentation, otherwise the code would break for ARMv6
when gcc gets fixed.

But the compiler can't assume the state of the U bit. I think it is
still legal on v6 to not support unaligned accesses, but on v7 it is
required. All the standard v6 ARM cores support it, but I'm not sure
about custom cores or if there are SOCs with buses that don't support
unaligned accesses properly.

The fact is that gcc assumes the A bit is cleared when generating code 
for ARMv7, period.  We need to conform our environment to gcc 
expectations, or always compile the decompressor using a lower 
architecture level than ARMv7.  My preference is the former.


Nicolas