[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

Tetsuyuki Kobayashi
2012-07-03T07:10:32Z

Hello, M?ns
Thank you for summarizing.

I am not against you.
I'm OK either "Allow unaligned access in U-Boot setting" or
"Specify compiler not to generate unaligned memory access"
or other.
I just want to solve hung-up by unaligned access.
I follow custodian's decision.

(2012/07/03 1:14), M?ns Rullg?rd wrote:
> Lucas Stach<dev@lynxeye.de> writes:
>
>> Am Montag, den 02.07.2012, 10:53 +0100 schrieb M?ns Rullg?rd:
>>> Tetsuyuki Kobayashi<koba@kmckk.co.jp> writes:
>>>
>>>> Recent compiler generates unaligned memory access in armv7 default.
>>>> But current U-Boot does not allow unaligned memory access, so it causes
>>>> data abort exception.
>>>> This patch add compile option "-mno-unaligned-access" if it is available.
>>>
>>> Why not allow unaligned accesses instead?
>>>
>> IMHO, our recent discussion showed that both ways are wrong.
>> "-mno-unaligned-access" works around misaligned data on the software
>> level, while allowing unaligned access does on the hardware level.
>>
>> What we really want is no unaligned access in U-Boot at all. Just
>> because "-mno-unaligned-access" is the default on ARMv5, we should not
>> consider it a gold standard.
>
> It's slightly more complicated than that. Data can be misaligned for a
> variety of reasons:
>
> 1. Errors in software.
> 2. Specified by a file format or communication protocol.
> 3. Deliberately misaligned by the compiler.
>
> Misaligned data of type 1 should of course be fixed properly, not worked
> around in any way.
>
> Type 2 happens all the time, and has to be dealt with one way or
> another. If the hardware supports unaligned accesses, this is usually
> faster than reading a byte at a time.
>
> When targeting ARMv6 and later, recent gcc versions have started issuing
> deliberate unaligned accesses where previously byte by byte accesses
> would have been done. This happens with "packed" structs and sometimes
> to write multiple smaller values at once, typically when
> zero-initialising things. These unaligned accesses are *good*. They
> make code smaller and faster.
>
> The real problem here is that u-boot is setting the strict alignment
> checking flag, invalidating the assumption of the compiler that the
> system allows unaligned accesses. For ARMv5 and earlier, setting this
> flag is usually advisable since it makes finding accidental unaligned
> accesses much easier.
>
> This was debated in the context of the kernel a while ago, ultimately
> leading to strict alignment being disabled for ARMv6 and up [1].
>
> [1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05
>