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, 30 insertions, 0 deletions

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);