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

From: Geert Uytterhoeven
Date: Wed Sep 17 2014 - 07:57:34 EST

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.

Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Question: Do we need the same protection in dbg_reset_notify()?
I can't trigger that case, as r8a7740 has only one CPU core.
arch/arm/kernel/hw_breakpoint.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index 4d963fb66e3f0bce..f55f25d4e4de7674 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -1038,8 +1038,21 @@ static struct notifier_block dbg_reset_nb = {
static int dbg_cpu_pm_notify(struct notifier_block *self, unsigned long action,
void *v)
- if (action == CPU_PM_EXIT)
+ if (action == CPU_PM_EXIT) {
+ /*
+ * We need to tread carefully here because DBGSWENABLE may be
+ * driven low on this core and there isn't an architected way to
+ * determine that.
+ */
+ register_undef_hook(&debug_reg_hook);
+ /*
+ * Reset the breakpoint resources. We assume that a halting
+ * debugger will leave the world in a nice state for us.
+ */
+ unregister_undef_hook(&debug_reg_hook);
+ }

return NOTIFY_OK;

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