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

Russell King - ARM Linux linux at arm.linux.org.uk
Thu Oct 11 09:41:30 EDT 2012

On Thu, Oct 11, 2012 at 08:31:47AM -0500, Rob Herring wrote:
> On 10/11/2012 08:09 AM, Russell King - ARM Linux wrote:
> > 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.
> This only affects v7 cores. It should not vary for v7 cores as unaligned
> access is a required feature. So how is it going to vary on v7 CPUs?
> We've got bigger problems if there are v7 cores that don't handle
> unaligned accesses.


Your patch may only affect v7 cores, but you've raised the issue of the
decompressor performing unaligned accesses in general.  Shall I re-repeat
my point over that or is the problem here going to finally sink in?

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