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

Russell King - ARM Linux
2012-10-11T13:09:56Z

On Thu, Oct 11, 2012 at 07:43:22AM -0500, Rob Herring wrote:
> The contents of this were already reviewed on this thread, so I sent this
> to the patch system and this was Russell's reply:

So that's why I couldn't find it - the mailing list thread has a different
subject line to the patch. Don't do that. Given the amount of list
traffic we have today, that's as good as not having been posted at all.

> > NAK for two reasons.
> >
> > 1. It hasn't been on the list (I can't find a match for "clear SCTLR.A"
> > in my mailbox)
> >
> > 2. The behaviour of unaligned accesses vary depending on CPU. Some
> > fix-up the access, others load the word and then rotate it. If we have
> > decompressors which perform unaligned accesses, we need to fix this
> > properly to avoid the CPU specific behaviour, rather than tweaking
> > control bits to hide the problem.
>
> I'm simply matching the behavior of the kernel itself. The A bit is cleared
> for v7 kernels and compilers only generate unaligned accesses for v7.
> Without this the initial state of the A bit is undefined as a bootloader
> could have cleared it already. We should document the required state or set
> it to what we want.

Irrespective of this, (2) still stands. Unaligned accesses in the
decompressor without a fixup (which will be very hard to provide)
will return different data depending on the CPU as I mention in point
2.

So, using unaligned accesses in the decompressor may not give expected
results in all cases, so they're best avoided.

And if they're avoided, then we don't care about the setting of the A
bit.