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

Russell King - ARM Linux
2012-11-05T11:13:46Z

On Mon, Nov 05, 2012 at 10:48:50AM +0000, Dave Martin wrote:
> On Thu, Oct 25, 2012 at 05:08:16PM +0200, Johannes Stezenbach wrote:
> > On Thu, Oct 25, 2012 at 09:25:06AM -0500, Rob Herring wrote:
> > > On 10/25/2012 09:16 AM, Johannes Stezenbach wrote:
> > > > On Thu, Oct 25, 2012 at 07:41:45AM -0500, Rob Herring wrote:
> > > >> On 10/25/2012 04:34 AM, Johannes Stezenbach wrote:
> > > >>> On Thu, Oct 11, 2012 at 07:43:22AM -0500, Rob Herring wrote:
> > > >>>
> > > >>>> 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.
> >
> > Well, I read the "...since Linux version 2.6.28" comment
> > in the gcc changes page in the way that they assume the
> > U-bit is set. (Although I'm not sure it really is???)
>
> Actually, the kernel checks the arch version and the U bit on boot,
> and chooses the appropriate setting for the A bit depending on the
> result. (See arch/arm/mm/alignment.c:alignment_init().)

That is in the kernel itself, _after_ the decompressor has run. It is
not relevant to any discussion about the decompressor.

> Currently, we depend on the CPU reset behaviour or firmware/
> bootloader to set the U bit for v6, but the behaviour should be
> correct either way, though unaligned accesses will obviously
> perform (much) better with U=1.

Will someone _PLEASE_ address my initial comments against this patch
in light of the fact that it's now been proven _NOT_ to be just a V7
issue, rather than everyone seemingly buring their heads in the sand
over this.

The fact is, unaligned accesses in the decompressor are *undefined* at
present.

> For v7, we should definitely use -munaligned-access where available
> (unless it's the default?)

No such option on my compiler - according to the manual I have, the only
option there is starting -munaligned is on SPARC for -munaligned-doubles.

However, I believe GCC does believe that unaligned accesses are fine on
V6 and above.