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

[Tetsuyuki Kobayashi]

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]