net: dsa: Mock-up driver

This patch adds support for a DSA mock-up driver which essentially does
the following:

- registers/unregisters 4 fixed PHYs to the slave network devices
- uses eth0 (configurable) as the master netdev
- registers the switch as a fixed MDIO device against the fixed MDIO bus
  at address 31
- includes dynamic debug prints for dsa_switch_ops functions that can be
  enabled to get call traces

This is a good way to test modular builds as well as exercise the DSA
APIs without requiring access to real hardware. This does not test the
data-path, although this could be added later on.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/dsa_loop_bdinfo.c

@@ -0,0 +1,34 @@
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/phy.h>
+#include <net/dsa.h>
+
+#include "dsa_loop.h"
+
+static struct dsa_loop_pdata dsa_loop_pdata = {
+	.cd = {
+		.port_names[0] = "lan1",
+		.port_names[1] = "lan2",
+		.port_names[2] = "lan3",
+		.port_names[3] = "lan4",
+		.port_names[DSA_LOOP_CPU_PORT] = "cpu",
+	},
+	.name = "DSA mockup driver",
+	.enabled_ports = 0x1f,
+	.netdev = "eth0",
+};
+
+static const struct mdio_board_info bdinfo = {
+	.bus_id	= "fixed-0",
+	.modalias = "dsa-loop",
+	.mdio_addr = 31,
+	.platform_data = &dsa_loop_pdata,
+};
+
+static int __init dsa_loop_bdinfo_init(void)
+{
+	return mdiobus_register_board_info(&bdinfo, 1);
+}
+arch_initcall(dsa_loop_bdinfo_init)
+
+MODULE_LICENSE("GPL");