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Subject: Re: -Wtautological-constant-compare in arch/s390/include/asm/page.h
To:     Nathan Chancellor <natechancellor@gmail.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
References: <20200208125714.GA9164@ubuntu-x2-xlarge-x86>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Mon, 10 Feb 2020 08:55:46 +0100
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On 08.02.20 13:57, Nathan Chancellor wrote:
> Hi all,
> 
> We noticed that you all added support for building s390 with clang,
> which is great! I have noticed a few warnings for which I will send
> patches but this one has me stumped.
> 
> In file included from ../lib/crypto/sha256.c:16:
> In file included from ../include/linux/module.h:13:
> In file included from ../include/linux/stat.h:19:
> In file included from ../include/linux/time.h:6:
> In file included from ../include/linux/seqlock.h:36:
> In file included from ../include/linux/spinlock.h:51:
> In file included from ../include/linux/preempt.h:78:
> In file included from ../arch/s390/include/asm/preempt.h:6:
> In file included from ../include/linux/thread_info.h:38:
> In file included from ../arch/s390/include/asm/thread_info.h:26:
> ../arch/s390/include/asm/page.h:45:6: warning: converting the result of '<<' to a boolean always evaluates to false [-Wtautological-constant-compare]
>         if (PAGE_DEFAULT_KEY)
>             ^
> ../arch/s390/include/asm/page.h:23:44: note: expanded from macro 'PAGE_DEFAULT_KEY'
> #define PAGE_DEFAULT_KEY        (PAGE_DEFAULT_ACC << 4)
>                                                   ^
> 1 warning generated.
> 
> PAGE_DEFAULT_PAGE is always 0, meaning this function never does what it
> is supposed to. Is this intentional? It seems that commit 0b642ede4796
> ("[PATCH] s390: default storage key") added this and it mentions that it
> can be overwritten at build time but I do not see any infrastructure for
> doing that. Any clarification that you can give so we can solve this
> warning would be much appreciated!

Yes, it is a debugging tool that we use from time to time. The user would then
change PAGE_DEFAULT_ACC in the header file when needed. It was not worth a config
option as normal users should not use it.