linux-kernel - Re: [PATCH] ARCv2: Add explcit unaligned access support (and ability to disable too)

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Date:   Mon, 4 Mar 2019 22:32:00 +0000
From:   Vineet Gupta <vineet.gupta1@...opsys.com>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Claudiu Zissulescu" <claudiu.zissulescu@...opsys.com>
Subject: Re: [PATCH] ARCv2: Add explcit unaligned access support (and
 ability to disable too)

On 2/28/19 11:24 AM, Vineet Gupta wrote:
> On 2/28/19 10:59 AM, Guenter Roeck wrote:
>>> Once the patch hits mainline you could use that. In the mean time I'll back out
>>> the "detector" as it might trip other people too.
>>>
>> Can you possibly use $(cc-option,-mno-unaligned-access), or would that
>> defeat the purpose ?
>>
> Indeed I was looking into Kconfig documentation and found something similar.
> However there's an anti-dependency. It is enabled by default and can only be
> disabled with $(cc-option,-mno-unaligned-access)
> So guess we will have to change  ARC_USE_UNALIGNED_MEM_ACCESS to
> ARC_*LACKS*_UNALIGNED_MEM_ACCESS
>
> I'll tinker a bit and post an update soon.

I got around to testing a patch with inverted option, unaligned access is default
and ARC_UNALIGNED_ACCESS_NONE disables it. This option can depend on
$(cc-option,-mno-unaligned-access). The issue however is "broken" gcc silently
ignoring -mno-unaligned-access. So this option can't be auto disable with affected
gcc. So this approach won't work. The only way out is remove the stringent check
and just wait for upstream gcc to be fixed.