[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

Tetsuyuki Kobayashi
2012-06-27T10:14:43Z

Hi Rob and all,

>>>
>>> Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
>>>> On 06/05/2012 11:47 AM, Lucas Stach wrote:
>>>>> Recent toolchains default to using the hardware feature for
>>>>> unaligned access on ARM v7, rather than doing the software
>>>>> fallback. According to ARM this is safe as all v7 implementations
>>>>> have to support this feature.
>>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
>>>>>
>>>>> To avoid CPU hangs when doing unaligned memory access, we have to
>>>>> turn off alignment checking in our CPU initialisation code.
>>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
>>>>
>>>> Does this behavior change trickle down to Linux/... too, or would
>>>> an OS completely re-initialize this state, and hence not be
>>>> affected?
>>>>
>>>
>>> Linux in particular does reinitialize this state and I expect any
>>> reasonable OS to do so.
>>
>> Then what is the point of enabling it on U-Boot? Does it fix some issue
>> whereby some mis-aligned piece of data cannot be properly aligned?
>>
>
> This is a new optimization feature in gcc 4.7 (and backported to some
> 4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):
>
> http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html
>
> http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/
>
> If you don't want to enable unaligned accesses, then
> "-mno-unaligned-access" needs to be added.
>

I verified it. Option "-mno-unaligned-access" works good.

include/mtd/cfi_flash.h

/* CFI standard query structure */
struct cfi_qry {
u8 qry[3];
u16 p_id; <-- unaligned!
...
} __attribute__((packed));

$ ${CROSS_COMPILE}gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.6.2 20110921 (release) [ARM/embedded-4_6-branch revision 182083]
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

=====================================================================
Compiled without --mno-unaligned-access
$ ${CROSS_COMPILE}objdump -d -S u-boot

info->vendor = le16_to_cpu(qry.p_id);
cc88: e3003a1c movw r3, #2588 ; 0xa1c
cc8c: e1dd11bb ldrh r1, [sp, #27] <-- this is unaligned access
cc90: ...
cc94: e18410b3 strh r1, [r4, r3]

=====================================================================
Compiled with --mno-unaligned-access
$ ${CROSS_COMPILE}objdump -d -S u-boot

info->vendor = le16_to_cpu(qry.p_id);
cce8: e5dd101c ldrb r1, [sp, #28] <--
ccec: e5dd301b ldrb r3, [sp, #27] <-- separated 2 byte accesses
ccf0: ...
ccf4: e1831401 orr r1, r3, r1, lsl #8
ccf8: e3003a1c movw r3, #2588 ; 0xa1c
ccfc: e18410b3 strh r1, [r4, r3]