linux-kernel - ARM: config issue with ftrace function graph tracer

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Message-ID: <20190112010139.GA5288@r2700x.localdomain>
Date:   Fri, 11 Jan 2019 18:01:39 -0700
From:   Jeremy Fertic <jeremyfertic@...il.com>
To:     Stefan Agner <stefan@...er.ch>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>, rostedt@...dmis.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: ARM: config issue with ftrace function graph tracer

I'm having a problem with the ftrace function graph tracer on a 32 bit arm
board (orangepi pc). A bisect points to the following commit:

f9b58e8c7d03 ("ARM: 8800/1: use choice for kernel unwinders")

Before this commit, if I use sunxi_defconfig and then menuconfig to enable
FTRACE and FUNCTION_TRACER then the function graph tracer works. With this
commit, and as of v5.0-rc1, doing the same as above results in a broken
function graph tracer and often an oops as well. The commit introduces a
choice group and it looks like it should default to UNWINDER_FRAME_POINTER
if FUNCTION_GRAPH_TRACER is enabled. FUNCTION_GRAPH_TRACER is enabled by
default when I enable FUNCTION_TRACER but this has no effect on the choice.
The choice always defaults to the other option which is UNWINDER_ARM. If I
manually choose UNWINDER_FRAME_POINTER then the function graph tracer works
fine.

Any idea why the default behaviour has changed?

Thanks,
Jeremy