From mboxrd@z Thu Jan 1 00:00:00 1970 From: robherring2@gmail.com (Rob Herring) Date: Thu, 11 Oct 2012 08:31:47 -0500 Subject: [PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores In-Reply-To: <20121011130955.GP4625@n2100.arm.linux.org.uk> References: <1349959402-24164-1-git-send-email-robherring2@gmail.com> <20121011130955.GP4625@n2100.arm.linux.org.uk> Message-ID: <5076CA43.9070503@gmail.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. Rob > 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. >