powerpc/xmon: Setup debugger hooks when first break-point is set

Presently sysrq key for xmon('x') is registered during kernel init
irrespective of the value of kernel param 'xmon'. Thus xmon is enabled
even if 'xmon=off' is passed on the kernel command line. However this
doesn't enable the kernel debugger hooks needed for instruction or
data breakpoints. Thus when a break-point is hit with xmon=off a
kernel oops of the form below is reported:

  Oops: Exception in kernel mode, sig: 5 [#1]
  < snip >
  Trace/breakpoint trap

To fix this the patch checks and enables debugger hooks when an
instruction or data break-point is set via xmon console.

Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
[mpe: Just printf directly, no need for static const char[]]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/xmon/xmon.c

@@ -1273,6 +1273,16 @@ static long check_bp_loc(unsigned long addr)
 	return 1;
 }
 
+/* Force enable xmon if not already enabled */
+static inline void force_enable_xmon(void)
+{
+	/* Enable xmon hooks if needed */
+	if (!xmon_on) {
+		printf("xmon: Enabling debugger hooks\n");
+		xmon_on = 1;
+	}
+}
+
 static char *breakpoint_help_string =
     "Breakpoint command usage:\n"
     "b                show breakpoints\n"
@@ -1315,6 +1325,8 @@ bpt_cmds(void)
 			dabr.address &= ~HW_BRK_TYPE_DABR;
 			dabr.enabled = mode | BP_DABR;
 		}
+
+		force_enable_xmon();
 		break;
 
 	case 'i':	/* bi - hardware instr breakpoint */
@@ -1335,6 +1347,7 @@ bpt_cmds(void)
 		if (bp != NULL) {
 			bp->enabled |= BP_CIABR;
 			iabr = bp;
+			force_enable_xmon();
 		}
 		break;
 #endif
@@ -1399,8 +1412,10 @@ bpt_cmds(void)
 		if (!check_bp_loc(a))
 			break;
 		bp = new_breakpoint(a);
-		if (bp != NULL)
+		if (bp != NULL) {
 			bp->enabled |= BP_TRAP;
+			force_enable_xmon();
+		}
 		break;
 	}
 }