MN10300: Emulate single stepping in KGDB on MN10300

Emulate single stepping in KGDB on MN10300 by way of temporary breakpoint
insertion.  These breakpoints are never actually seen by KGDB, and will overlay
KGDB's own breakpoints.

The breakpoints are removed by switch_to() and reinstalled on switching back so
that if preemption occurs, the preempting task doesn't hit them (though it will
still hit KGDB's regular breakpoints).  If KGDB is reentered for any reason,
then the single step breakpoint is completely erased and must be set again by
the debugger.

We take advantage of the fact that KGDB will effectively halt all other CPUs
whilst this CPU is single-stepping to avoid SMP problems.

If the single-stepping task is preempted and killed without KGDB being
reinvoked, then the breakpoint(s) will be cleared and KGDB will be jumped back
into.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 110 insertions, 1 deletions

diff --git a/arch/mn10300/kernel/switch_to.S b/arch/mn10300/kernel/switch_to.S
index 9074d0fb8788..de3e74fc9ea0 100644
--- a/arch/mn10300/kernel/switch_to.S
+++ b/arch/mn10300/kernel/switch_to.S
@@ -39,11 +39,17 @@ ENTRY(__switch_to)
 
 	# save prev context
 	mov	__switch_back,d0
-	mov	d0,(THREAD_PC,a0)
 	mov	sp,a2
 	mov	a2,(THREAD_SP,a0)
 	mov	a3,(THREAD_A3,a0)
 
+#ifdef CONFIG_KGDB
+	btst	0xff,(kgdb_single_step)
+	bne	__switch_to__lift_sstep_bp
+__switch_to__continue:
+#endif
+	mov	d0,(THREAD_PC,a0)
+
 	mov	(THREAD_A3,a1),a3
 	mov	(THREAD_SP,a1),a2
 
@@ -68,3 +74,106 @@ ENTRY(__switch_to)
 __switch_back:
 	and	~EPSW_NMID,epsw
 	ret	[d2,d3,a2,a3,exreg1],32
+
+#ifdef CONFIG_KGDB
+###############################################################################
+#
+# Lift the single-step breakpoints when the task being traced is switched out
+# A0 = prev
+# A1 = next
+#
+###############################################################################
+__switch_to__lift_sstep_bp:
+	add	-12,sp
+	mov	a0,e4
+	mov	a1,e5
+
+	# Clear the single-step flag to prevent us coming this way until we get
+	# switched back in
+	bclr	0xff,(kgdb_single_step)
+
+	# Remove first breakpoint
+	mov	(kgdb_sstep_bp_addr),a2
+	cmp	0,a2
+	beq	1f
+	movbu	(kgdb_sstep_bp),d0
+	movbu	d0,(a2)
+#if defined(CONFIG_MN10300_CACHE_FLUSH_ICACHE) || defined(CONFIG_MN10300_CACHE_INV_ICACHE)
+	mov	a2,d0
+	mov	a2,d1
+	add	1,d1
+	calls	flush_icache_range
+#endif
+1:
+
+	# Remove second breakpoint
+	mov	(kgdb_sstep_bp_addr+4),a2
+	cmp	0,a2
+	beq	2f
+	movbu	(kgdb_sstep_bp+1),d0
+	movbu	d0,(a2)
+#if defined(CONFIG_MN10300_CACHE_FLUSH_ICACHE) || defined(CONFIG_MN10300_CACHE_INV_ICACHE)
+	mov	a2,d0
+	mov	a2,d1
+	add	1,d1
+	calls	flush_icache_range
+#endif
+2:
+
+	# Change the resumption address and return
+	mov	__switch_back__reinstall_sstep_bp,d0
+	mov	e4,a0
+	mov	e5,a1
+	add	12,sp
+	bra	__switch_to__continue
+
+###############################################################################
+#
+# Reinstall the single-step breakpoints when the task being traced is switched
+# back in (A1 points to the new thread_struct).
+#
+###############################################################################
+__switch_back__reinstall_sstep_bp:
+	add	-12,sp
+	mov	a0,e4			# save the return value
+	mov	0xff,d3
+
+	# Reinstall first breakpoint
+	mov	(kgdb_sstep_bp_addr),a2
+	cmp	0,a2
+	beq	1f
+	movbu	(a2),d0
+	movbu	d0,(kgdb_sstep_bp)
+	movbu	d3,(a2)
+#if defined(CONFIG_MN10300_CACHE_FLUSH_ICACHE) || defined(CONFIG_MN10300_CACHE_INV_ICACHE)
+	mov	a2,d0
+	mov	a2,d1
+	add	1,d1
+	calls	flush_icache_range
+#endif
+1:
+
+	# Reinstall second breakpoint
+	mov	(kgdb_sstep_bp_addr+4),a2
+	cmp	0,a2
+	beq	2f
+	movbu	(a2),d0
+	movbu	d0,(kgdb_sstep_bp+1)
+	movbu	d3,(a2)
+#if defined(CONFIG_MN10300_CACHE_FLUSH_ICACHE) || defined(CONFIG_MN10300_CACHE_INV_ICACHE)
+	mov	a2,d0
+	mov	a2,d1
+	add	1,d1
+	calls	flush_icache_range
+#endif
+2:
+
+	mov	d3,(kgdb_single_step)
+
+	# Restore the return value (the previous thread_struct pointer)
+	mov	e4,a0
+	mov	a0,d0
+	add	12,sp
+	bra	__switch_back
+
+#endif /* CONFIG_KGDB */