From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Mon, 5 Nov 2012 11:13:46 +0000 Subject: [PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores In-Reply-To: <20121105104839.GA2005@linaro.org> References: <1349959402-24164-1-git-send-email-robherring2@gmail.com> <20121025093411.GA32662@sig21.net> <50893389.2090002@gmail.com> <20121025141645.GA16962@sig21.net> <50894BC2.5050706@gmail.com> <20121025150816.GA3874@sig21.net> <20121105104839.GA2005@linaro.org> Message-ID: <20121105111346.GF28327@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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.