net/p9: load default transports

Now that all transports are split into modules it may happen that no
transports are registered when v9fs_get_default_trans() is called.
When that is the case try to load more transports from modules.

Link: https://lkml.kernel.org/r/20211103193823.111007-5-linux@weissschuh.netSigned-off-by: Thomas Weißschuh <linux@weissschuh.net>
[Dominique: constify v9fs_get_trans_by_name argument as per patch1v2]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

include/net/9p/transport.h

@@ -54,7 +54,7 @@ struct p9_trans_module {
 
 void v9fs_register_trans(struct p9_trans_module *m);
 void v9fs_unregister_trans(struct p9_trans_module *m);
-struct p9_trans_module *v9fs_get_trans_by_name(char *s);
+struct p9_trans_module *v9fs_get_trans_by_name(const char *s);
 struct p9_trans_module *v9fs_get_default_trans(void);
 void v9fs_put_trans(struct p9_trans_module *m);
 

net/9p/mod.c

@@ -83,7 +83,7 @@ void v9fs_unregister_trans(struct p9_trans_module *m)
 }
 EXPORT_SYMBOL(v9fs_unregister_trans);
 
-static struct p9_trans_module *_p9_get_trans_by_name(char *s)
+static struct p9_trans_module *_p9_get_trans_by_name(const char *s)
 {
 	struct p9_trans_module *t, *found = NULL;
 
@@ -106,7 +106,7 @@ static struct p9_trans_module *_p9_get_trans_by_name(char *s)
  * @s: string identifying transport
  *
  */
-struct p9_trans_module *v9fs_get_trans_by_name(char *s)
+struct p9_trans_module *v9fs_get_trans_by_name(const char *s)
 {
 	struct p9_trans_module *found = NULL;
 
@@ -123,6 +123,10 @@ struct p9_trans_module *v9fs_get_trans_by_name(char *s)
 }
 EXPORT_SYMBOL(v9fs_get_trans_by_name);
 
+static const char * const v9fs_default_transports[] = {
+	"virtio", "tcp", "fd", "unix", "xen", "rdma",
+};
+
 /**
  * v9fs_get_default_trans - get the default transport
  *
@@ -131,6 +135,7 @@ EXPORT_SYMBOL(v9fs_get_trans_by_name);
 struct p9_trans_module *v9fs_get_default_trans(void)
 {
 	struct p9_trans_module *t, *found = NULL;
+	int i;
 
 	spin_lock(&v9fs_trans_lock);
 
@@ -148,6 +153,10 @@ struct p9_trans_module *v9fs_get_default_trans(void)
 			}
 
 	spin_unlock(&v9fs_trans_lock);
+
+	for (i = 0; !found && i < ARRAY_SIZE(v9fs_default_transports); i++)
+		found = v9fs_get_trans_by_name(v9fs_default_transports[i]);
+
 	return found;
 }
 EXPORT_SYMBOL(v9fs_get_default_trans);