[ 3.8.y.z extended stable ] Patch "arm64: debug: clear mdscr_el1 instead of taking the OS lock" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    arm64: debug: clear mdscr_el1 instead of taking the OS lock

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From aed0d169a09ab2b83e9dcc1ce91c2f94799fe0bf Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon at arm.com>
Date: Wed, 8 May 2013 17:01:12 +0100
Subject: arm64: debug: clear mdscr_el1 instead of taking the OS lock

commit 9c413e25d95fd3c0abe23eadad66191b22baa549 upstream.

During boot, we take the debug OS lock before interrupts are enabled.
This is required to prevent clearing of PSTATE.D on the interrupt entry
path, which could result in spurious debug exceptions before we've got
round to resetting things like the hardware breakpoints registers to a
sane state.

A problem with this approach is that taking the OS lock prevents an
external JTAG debugger from debugging the system, which is especially
irritating during boot, where JTAG debugging can be most useful.

This patch clears mdscr_el1 rather than taking the lock, clearing the
MDE and KDE bits and preventing self-hosted hardware debug exceptions
from occurring.

Tested-by: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm64/kernel/debug-monitors.c | 2 --
 arch/arm64/mm/proc.S               | 3 +--
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 0c3ba9f..f4726dc 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -136,8 +136,6 @@ void disable_debug_monitors(enum debug_el el)
  */
 static void clear_os_lock(void *unused)
 {
-	asm volatile("msr mdscr_el1, %0" : : "r" (0));
-	isb();
 	asm volatile("msr oslar_el1, %0" : : "r" (0));
 	isb();
 }
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index f1d8b9b..a82ae88 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -119,8 +119,7 @@ ENTRY(__cpu_setup)

 	mov	x0, #3 << 20
 	msr	cpacr_el1, x0			// Enable FP/ASIMD
-	mov	x0, #1
-	msr	oslar_el1, x0			// Set the debug OS lock
+	msr	mdscr_el1, xzr			// Reset mdscr_el1
 	tlbi	vmalle1is			// invalidate I + D TLBs
 	/*
 	 * Memory region attributes for LPAE:
--
1.8.1.2