3 files changed, 59 insertions, 10 deletions

diff --git a/Documentation/power/interface.txt b/Documentation/power/interface.txt
index 4117802af0f..a66bec222b1 100644
--- a/Documentation/power/interface.txt
+++ b/Documentation/power/interface.txt
@@ -52,3 +52,18 @@ suspend image will be as small as possible.
 
 Reading from this file will display the current image size limit, which
 is set to 500 MB by default.
+
+/sys/power/pm_trace controls the code which saves the last PM event point in
+the RTC across reboots, so that you can debug a machine that just hangs
+during suspend (or more commonly, during resume).  Namely, the RTC is only
+used to save the last PM event point if this file contains '1'.  Initially it
+contains '0' which may be changed to '1' by writing a string representing a
+nonzero integer into it.
+
+To use this debugging feature you should attempt to suspend the machine, then
+reboot it and run
+
+        dmesg -s 1000000 | grep 'hash matches'
+
+CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
+set to a random invalid time after a resume.
diff --git a/include/linux/resume-trace.h b/include/linux/resume-trace.h
index a376bd4ade3..81e9299ca14 100644
--- a/include/linux/resume-trace.h
+++ b/include/linux/resume-trace.h
@@ -3,21 +3,25 @@
 
 #ifdef CONFIG_PM_TRACE
 
+extern int pm_trace_enabled;
+
 struct device;
 extern void set_trace_device(struct device *);
 extern void generate_resume_trace(void *tracedata, unsigned int user);
 
 #define TRACE_DEVICE(dev) set_trace_device(dev)
-#define TRACE_RESUME(user) do {                         \
-        void *tracedata;                                \
-        asm volatile("movl $1f,%0\n"                    \
-                ".section .tracedata,\"a\"\n"           \
-                "1:\t.word %c1\n"                       \
-                "\t.long %c2\n"                         \
-                ".previous"                             \
-                :"=r" (tracedata)                       \
-                : "i" (__LINE__), "i" (__FILE__));      \
-        generate_resume_trace(tracedata, user);         \
+#define TRACE_RESUME(user) do {                                 \
+        if (pm_trace_enabled) {                                 \
+                void *tracedata;                                \
+                asm volatile("movl $1f,%0\n"                    \
+                        ".section .tracedata,\"a\"\n"           \
+                        "1:\t.word %c1\n"                       \
+                        "\t.long %c2\n"                         \
+                        ".previous"                             \
+                        :"=r" (tracedata)                       \
+                        : "i" (__LINE__), "i" (__FILE__));      \
+                generate_resume_trace(tracedata, user);         \
+        }                                                       \
 } while (0)
 
 #else
diff --git a/kernel/power/main.c b/kernel/power/main.c
index 4d403323a7b..873228c71da 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -17,6 +17,7 @@
 #include <linux/pm.h>
 #include <linux/console.h>
 #include <linux/cpu.h>
+#include <linux/resume-trace.h>
 
 #include "power.h"
 
@@ -281,10 +282,39 @@ static ssize_t state_store(struct subsystem * subsys, const char * buf, size_t n
 
 power_attr(state);
 
+#ifdef CONFIG_PM_TRACE
+int pm_trace_enabled;
+
+static ssize_t pm_trace_show(struct subsystem * subsys, char * buf)
+{
+        return sprintf(buf, "%d\n", pm_trace_enabled);
+}
+
+static ssize_t
+pm_trace_store(struct subsystem * subsys, const char * buf, size_t n)
+{
+        int val;
+
+        if (sscanf(buf, "%d", &val) == 1) {
+                pm_trace_enabled = !!val;
+                return n;
+        }
+        return -EINVAL;
+}
+
+power_attr(pm_trace);
+
+static struct attribute * g[] = {
+        &state_attr.attr,
+        &pm_trace_attr.attr,
+        NULL,
+};
+#else
 static struct attribute * g[] = {
         &state_attr.attr,
         NULL,
 };
+#endif /* CONFIG_PM_TRACE */
 
 static struct attribute_group attr_group = {
         .attrs = g,