From: robherring2@gmail•com (Rob Herring)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores
Date: Mon, 05 Nov 2012 07:48:37 -0600 [thread overview]
Message-ID: <5097C3B5.4080406@gmail.com> (raw)
In-Reply-To: <20121105111346.GF28327@n2100.arm.linux.org.uk>
On 11/05/2012 05:13 AM, Russell King - ARM Linux wrote:
> 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.
I tried adding -munaligned-accesses on a v6 build and still get byte
accesses rather than unaligned word accesses. So this does seem to be a
v7 only issue based on what gcc will currently produce. Copying Michael
Hope who can hopefully provide some insight on why v6 unaligned accesses
are not enabled.
> 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.
It's only added in 4.7 and backported to Linaro 4.6.3.
Rob
> However, I believe GCC does believe that unaligned accesses are fine on
> V6 and above.
>
next prev parent reply other threads:[~2012-11-05 13:48 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-05 1:23 [PATCH v2 0/5] Use more asm-generic headers Rob Herring
2012-08-05 1:23 ` [PATCH v2 1/5] ARM: use generic version of identical asm headers Rob Herring
2012-08-05 1:23 ` [PATCH v2 2/5] ARM: add strstr declaration for decompressors Rob Herring
2012-08-05 1:23 ` [PATCH v2 3/5] ARM: use generic unaligned.h Rob Herring
2012-10-08 16:43 ` Shawn Guo
2012-10-08 20:34 ` Rob Herring
2012-10-08 23:28 ` Shawn Guo
2012-10-09 2:27 ` Rob Herring
2012-10-09 2:45 ` Shawn Guo
2012-10-09 4:01 ` Nicolas Pitre
2012-10-10 13:29 ` Rob Herring
2012-10-10 13:57 ` Nicolas Pitre
2012-10-11 12:43 ` [PATCH] ARM: decompressor: clear SCTLR.A bit for v7 cores Rob Herring
2012-10-11 13:09 ` Russell King - ARM Linux
2012-10-11 13:31 ` Rob Herring
2012-10-11 13:41 ` Russell King - ARM Linux
2012-10-11 15:44 ` Nicolas Pitre
2012-10-23 20:32 ` Gregory CLEMENT
2012-10-25 12:07 ` Russell King - ARM Linux
2012-10-11 13:59 ` Russell King - ARM Linux
2012-10-11 15:58 ` Nicolas Pitre
2012-10-25 9:34 ` Johannes Stezenbach
2012-10-25 12:41 ` Rob Herring
2012-10-25 13:30 ` Arnd Bergmann
2012-10-25 14:16 ` Johannes Stezenbach
2012-10-25 14:25 ` Rob Herring
2012-10-25 15:02 ` Nicolas Pitre
2012-10-25 15:08 ` Johannes Stezenbach
2012-11-05 10:48 ` Dave Martin
2012-11-05 11:13 ` Russell King - ARM Linux
2012-11-05 13:02 ` Dave Martin
2012-11-05 13:43 ` Johannes Stezenbach
2012-11-05 15:39 ` Russell King - ARM Linux
2012-11-05 16:13 ` Nicolas Pitre
2012-11-05 17:26 ` Dave Martin
2012-11-05 17:44 ` Rob Herring
2012-11-05 20:02 ` Nicolas Pitre
2012-11-05 17:46 ` Nicolas Pitre
2012-11-05 13:48 ` Rob Herring [this message]
2012-11-05 22:02 ` Michael Hope
2013-02-20 14:56 ` Johannes Stezenbach
2013-02-20 15:07 ` Johannes Stezenbach
2012-08-05 1:23 ` [PATCH v2 4/5] ARM: use generic termios.h Rob Herring
2012-08-05 1:24 ` [PATCH v2 5/5] ARM: convert asm/irqflags.h to use asm-generic/irqflags.h Rob Herring
2012-08-05 9:34 ` [PATCH v2 0/5] Use more asm-generic headers Thomas Petazzoni
2012-08-06 14:35 ` Arnd Bergmann
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