Currently the xmon debugger is set only via kernel boot command-line.
It's disabled by default, and can be enabled with "xmon=on" on the
command-line. Also, xmon may be accessed via sysrq mechanism.
But we cannot enable/disable xmon in runtime, it needs kernel reload.

This patch introduces a debugfs entry for xmon, allowing user to query
its current state and change it if desired. Basically, the "xmon" file
to read from/write to is under the debugfs mount point, on powerpc
directory. It's a simple attribute, value 0 meaning xmon is disabled
and value 1 the opposite. Writing these states to the file will take
immediate effect in the debugger.

Signed-off-by: Guilherme G. Piccoli <redacted>
---
v3: logic improved based in the changes made on patch 1.

v2: dropped the custom parser by using simple attributes [mpe suggestion].


 arch/powerpc/xmon/xmon.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index f1fcfa8..764ea62 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c

@@ -29,6 +29,10 @@
 #include <linux/nmi.h>
 #include <linux/ctype.h>

+#ifdef CONFIG_DEBUG_FS
+#include <linux/debugfs.h>
+#endif
+
 #include <asm/ptrace.h>
 #include <asm/string.h>
 #include <asm/prom.h>

@@ -3275,6 +3279,33 @@ static int __init setup_xmon_sysrq(void)
 device_initcall(setup_xmon_sysrq);
 #endif /* CONFIG_MAGIC_SYSRQ */

+#ifdef CONFIG_DEBUG_FS
+static int xmon_dbgfs_set(void *data, u64 val)
+{
+	xmon_off = !val;