linux-kernel - gcc7 log2 compile issues in kernel/time/timekeeping.c

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Date:   Thu, 23 Feb 2017 10:43:31 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: gcc7 log2 compile issues in kernel/time/timekeeping.c

Hi,

Fedora was previously carrying a workaround for a gcc7 issue reported
on arm64 http://lists.infradead.org/pipermail/linux-arm-kernel/2016-October/461597.html.
The workaround got rid of __ilog2_NaN. I dropped the patch this morning
because a proper fix (29905b52fad0 ("log2: make order_base_2() behave
correctly on const input value zero")) was merged. This fixed the arm64
problem linked in the thread but there seems to be another issue in
timekeeping.c:

/kernel/time/timekeeping.c:2051: undefined reference to `____ilog2_NaN'

Fedora enables CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE so I think the
compiler is calculating a possible constant of 0 once again.

Any ideas about a proper fix? The configuration used is attached.
I'm going to bring back the workaround in Fedora for now.

Thanks,
Laura

View attachment "kernel-4.11.0-x86_64.config" of type "text/plain" (185235 bytes)