linux-kernel - -Wtautological-constant-compare in arch/s390/include/asm/page.h

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Message-ID: <20200208125714.GA9164@ubuntu-x2-xlarge-x86>
Date:   Sat, 8 Feb 2020 05:57:14 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: -Wtautological-constant-compare in arch/s390/include/asm/page.h

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!

Cheers,
Nathan