st: allow debug output to be enabled or disabled via sysfs

Change st driver to allow enabling or disabling debug output
via sysfs file /sys/bus/scsi/drivers/st/debug_flag.

Previously the only way to enable debug output was:

1. loading the driver with the module parameter debug_flag=1
2. an ioctl call (this method was also the only way to dynamically
disable debug output).

To use the ioctl you need a second tape drive (if you are
actively testing the first tape drive) since a second process
cannot open the first tape drive if it is in use.

The this change is only functional if the value of the macro
DEBUG in st.c is a non-zero value (which it is by default).
Signed-off-by: Shane Seymour <shane.seymour@hpe.com>
Reviewed-by: Laurence Oberman <oberman.l@gmail.com>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

Documentation/ABI/testing/sysfs-driver-st

+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.

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

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);