ocfs2_dlmfs: Add capabilities parameter.

Over time, dlmfs has added some features that were not part of the
initial ABI.  Unfortunately, some of these features are not detectable
via standard usage.  For example, Linux's default poll always returns
POLLIN, so there is no way for a caller of poll(2) to know when dlmfs
added poll support.  Instead, we provide this list of new capabilities.

Capabilities is a read-only attribute.  We do it as a module parameter
so we can discover it whether dlmfs is built in, loaded, or even not
loaded (via modinfo).

The ABI features are local to this machine's dlmfs mount.  This is
distinct from the locking protocol, which is concerned with inter-node
interaction.
Signed-off-by: NJoel Becker <joel.becker@oracle.com>

fs/ocfs2/dlm/dlmfs.c

@@ -81,6 +81,42 @@ static const struct dlm_protocol_version user_locking_protocol = {
 	.pv_minor = OCFS2_LOCKING_PROTOCOL_MINOR,
 };
 
+
+/*
+ * These are the ABI capabilities of dlmfs.
+ *
+ * Over time, dlmfs has added some features that were not part of the
+ * initial ABI.  Unfortunately, some of these features are not detectable
+ * via standard usage.  For example, Linux's default poll always returns
+ * POLLIN, so there is no way for a caller of poll(2) to know when dlmfs
+ * added poll support.  Instead, we provide this list of new capabilities.
+ *
+ * Capabilities is a read-only attribute.  We do it as a module parameter
+ * so we can discover it whether dlmfs is built in, loaded, or even not
+ * loaded.
+ *
+ * The ABI features are local to this machine's dlmfs mount.  This is
+ * distinct from the locking protocol, which is concerned with inter-node
+ * interaction.
+ */
+#define DLMFS_CAPABILITIES ""
+extern int param_set_dlmfs_capabilities(const char *val,
+					struct kernel_param *kp)
+{
+	printk(KERN_ERR "%s: readonly parameter\n", kp->name);
+	return -EINVAL;
+}
+static int param_get_dlmfs_capabilities(char *buffer,
+					struct kernel_param *kp)
+{
+	return strlcpy(buffer, DLMFS_CAPABILITIES,
+		       strlen(DLMFS_CAPABILITIES) + 1);
+}
+module_param_call(capabilities, param_set_dlmfs_capabilities,
+		  param_get_dlmfs_capabilities, NULL, 0444);
+MODULE_PARM_DESC(capabilities, DLMFS_CAPABILITIES);
+
+
 /*
  * decodes a set of open flags into a valid lock level and a set of flags.
  * returns < 0 if we have invalid flags