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>

1 files changed, 12 insertions, 0 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.