3 files changed, 45 insertions, 1 deletions

diff --git a/Documentation/ABI/testing/sysfs-driver-st b/Documentation/ABI/testing/sysfs-driver-st
new file mode 100644
index 000000000000..ba5d77008a85
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-driver-st
@@ -0,0 +1,12 @@
+What:           /sys/bus/scsi/drivers/st/debug_flag
+Date:           October 2015
+Kernel Version: ?.?
+Contact:        shane.seymour@hpe.com
+Description:
+                This file allows you to turn debug output from the st driver
+                off if you write a '0' to the file or on if you write a '1'.
+                Note that debug output requires that the module be compiled
+                with the #define DEBUG set to a non-zero value (this is the
+                default). If DEBUG is set to 0 then this file will not
+                appear in sysfs as its presence is conditional upon debug
+                output support being compiled into the module.
diff --git a/Documentation/scsi/st.txt b/Documentation/scsi/st.txt
index f29fa550665a..b3211af63b79 100644
--- a/Documentation/scsi/st.txt
+++ b/Documentation/scsi/st.txt
@@ -569,7 +569,9 @@ Debugging code is now compiled in by default but debugging is turned off
 with the kernel module parameter debug_flag defaulting to 0.  Debugging
 can still be switched on and off with an ioctl.  To enable debug at
 module load time add debug_flag=1 to the module load options, the
-debugging output is not voluminous.
+debugging output is not voluminous. Debugging can also be enabled
+and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
+file /sys/bus/scsi/drivers/st/debug_flag.
 
 If the tape seems to hang, I would be very interested to hear where
 the driver is waiting. With the command 'ps -l' you can see the state
diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index b37b9b00c4b4..e0a1e52a04e7 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -4452,11 +4452,41 @@ static ssize_t version_show(struct device_driver *ddd, char *buf)
 }
 static DRIVER_ATTR_RO(version);
 
+#if DEBUG
+static ssize_t debug_flag_store(struct device_driver *ddp,
+        const char *buf, size_t count)
+{
+/* We only care what the first byte of the data is the rest is unused.
+ * if it's a '1' we turn on debug and if it's a '0' we disable it. All
+ * other values have -EINVAL returned if they are passed in.
+ */
+        if (count > 0) {
+                if (buf[0] == '0') {
+                        debugging = NO_DEBUG;
+                        return count;
+                } else if (buf[0] == '1') {
+                        debugging = 1;
+                        return count;
+                }
+        }
+        return -EINVAL;
+}
+
+static ssize_t debug_flag_show(struct device_driver *ddp, char *buf)
+{
+        return scnprintf(buf, PAGE_SIZE, "%d\n", debugging);
+}
+static DRIVER_ATTR_RW(debug_flag);
+#endif
+
 static struct attribute *st_drv_attrs[] = {
         &driver_attr_try_direct_io.attr,
         &driver_attr_fixed_buffer_size.attr,
         &driver_attr_max_sg_segs.attr,
         &driver_attr_version.attr,
+#if DEBUG
+        &driver_attr_debug_flag.attr,
+#endif
         NULL,
 };
 ATTRIBUTE_GROUPS(st_drv);