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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 26 Sep 2018 09:25:26 +0200
Message-ID: <CAK8P3a3naNgsWRLCyMbACntte4DOaYz3tzP72SMZGDzaUoZtPQ@mail.gmail.com>
Subject: Re: [PATCH v2] ARM: use choice for kernel unwinders
To:     Stefan Agner <stefan@agner.ch>
Cc:     Russell King - ARM Linux <linux@armlinux.org.uk>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Sep 25, 2018 at 11:20 PM Stefan Agner <stefan@agner.ch> wrote:
>
> While in theory multiple unwinders could be compiled in, it does
> not make sense in practise. Use a choice to make the unwinder
> selection mutually exclusive and mandatory.
>
> Already before this commit it has not been possible to deselect
> FRAME_POINTER. Remove the obsolete comment.
>
> Furthermore, to produce a meaningful backtrace with FRAME_POINTER
> enabled the kernel needs a specific function prologue:
>     mov    ip, sp
>     stmfd    sp!, {fp, ip, lr, pc}
>     sub    fp, ip, #4
>
> To get to the required prologue gcc uses apcs and no-sched-prolog.
> This compiler options are not available on clang, and clang is not
> able to generate the required prologue. Make the FRAME_POINTER
> config symbol depending on !clang.
>
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Stefan Agner <stefan@agner.ch>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>

I've tested an earlier version of this patch on my randconfig builder
without issues.

       Arnd