Re: [PATCH] ARM: hw_breakpoint: Trap undef instruction exceptions on wake-up

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Sep 17, 2014 at 01:57:16PM +0200, Geert Uytterhoeven wrote:
> If power area D4, which contains the Coresight-ETM hardware block, is
> powered down on R-Mobile A1 (r8a7740), the kernel crashes when
> suspending from s2ram with:
> 
>     Internal error: Oops - undefined instruction: 0 [#1] ARM
> 
> This happens because dbg_cpu_pm_notify() calls reset_ctrl_regs(), which
> can't access the debug registers as the debug module is powered down.
> 
> Protect the call to reset_ctrl_regs() by an undefined instruction hook,
> like was done in commit 0d352e3d006c9589 ("ARM: hw_breakpoint: trap
> undef instruction exceptions in reset_ctrl_regs") for another caller.

I'd prefer a better solution to this (such as tracking whether the ETM
block is powered down).  This feels very much like a hack than a good
solution.

Also, the undefined instruction hook was not supposed to be used in this
way - certainly *not* dynamically like this.  While the lookup of the
undef handler is done in a safe atomic way, calling the resulting
function is outside of the locks.

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