[AVR32] Follow the rules when dealing with the OCD system

The current debug trap handling code does a number of things that are
illegal according to the AVR32 Architecture manual. Most importantly,
it may try to schedule from Debug Mode, thus clearing the D bit, which
can lead to "undefined behaviour".

It seems like this works in most cases, but several people have
observed somewhat unstable behaviour when debugging programs,
including soft lockups. So there's definitely something which is not
right with the existing code.

The new code will never schedule from Debug mode, it will always exit
Debug mode with a "retd" instruction, and if something not running in
Debug mode needs to do something debug-related (like doing a single
step), it will enter debug mode through a "breakpoint" instruction.
The monitor code will then return directly to user space, bypassing
its own saved registers if necessary (since we don't actually care
about the trapped context, only the one that came before.)

This adds three instructions to the common exception handling code,
including one branch. It does not touch super-hot paths like the TLB
miss handler.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/include/asm-avr32/thread_info.h b/include/asm-avr32/thread_info.h
index 67e7aae..184b574 100644
--- a/include/asm-avr32/thread_info.h
+++ b/include/asm-avr32/thread_info.h
@@ -25,6 +25,11 @@ struct thread_info {
 	unsigned long		flags;		/* low level flags */
 	__u32			cpu;
 	__s32			preempt_count;	/* 0 => preemptable, <0 => BUG */
+	__u32			rar_saved;	/* return address... */
+	__u32			rsr_saved;	/* ...and status register
+						   saved by debug handler
+						   when setting up
+						   trampoline */
 	struct restart_block	restart_block;
 	__u8			supervisor_stack[0];
 };
@@ -78,8 +83,8 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NEED_RESCHED        2       /* rescheduling necessary */
 #define TIF_POLLING_NRFLAG      3       /* true if poll_idle() is polling
 					   TIF_NEED_RESCHED */
-#define TIF_BREAKPOINT		4	/* true if we should break after return */
-#define TIF_SINGLE_STEP		5	/* single step after next break */
+#define TIF_BREAKPOINT		4	/* enter monitor mode on return */
+#define TIF_SINGLE_STEP		5	/* single step in progress */
 #define TIF_MEMDIE		6
 #define TIF_RESTORE_SIGMASK	7	/* restore signal mask in do_signal */
 #define TIF_CPU_GOING_TO_SLEEP	8	/* CPU is entering sleep 0 mode */
@@ -89,7 +94,6 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_POLLING_NRFLAG	(1 << TIF_POLLING_NRFLAG)
-#define _TIF_BREAKPOINT		(1 << TIF_BREAKPOINT)
 #define _TIF_SINGLE_STEP	(1 << TIF_SINGLE_STEP)
 #define _TIF_MEMDIE		(1 << TIF_MEMDIE)
 #define _TIF_RESTORE_SIGMASK	(1 << TIF_RESTORE_SIGMASK)
@@ -108,6 +112,6 @@ static inline struct thread_info *current_thread_info(void)
 /* work to do on any return to userspace */
 #define _TIF_ALLWORK_MASK	(_TIF_WORK_MASK | (1 << TIF_SYSCALL_TRACE))
 /* work to do on return from debug mode */
-#define _TIF_DBGWORK_MASK	(_TIF_WORK_MASK | (1 << TIF_SINGLE_STEP))
+#define _TIF_DBGWORK_MASK	(_TIF_WORK_MASK & ~(1 << TIF_BREAKPOINT))
 
 #endif /* __ASM_AVR32_THREAD_INFO_H */