net: mdio: access c22 registers via debugfs

Provide a debugging interface to read and write MDIO registers directly
without the need for a device driver.

This is extremely useful when debugging switch hardware and phy hardware
issues. The interface provides proper locking for communication that
consists of a sequence of MDIO read/write commands.

The interface binds directly to the MDIO bus abstraction in order to
provide support for all devices whether there's a hardware driver for
them or not. Registers are written by writing address, offset, and
value in hex, separated by colon. Registeres are read by writing only
address and offset, then reading the value.

It can be easily tested using `socat`:

    # socat - /sys/kernel/debug/mdio/f802c000.ethernet-ffffffff/control

Example: Reading address 0x00 offset 0x00, value is 0x3000

    Input: 00:00
    Output: 3000

Example: Writing address 0x00 offset 0x00, value 0x2100

    Input: 00:00:2100

Signed-off-by: Pavel Šimerda <code@simerda.eu>

drivers/net/phy/Makefile

@@ -17,6 +17,7 @@ libphy-y			+= $(mdio-bus-y)
 else
 obj-$(CONFIG_MDIO_DEVICE)	+= mdio-bus.o
 endif
+obj-$(CONFIG_MDIO_DEVICE)	+= mdio-debugfs.o
 obj-$(CONFIG_MDIO_DEVRES)	+= mdio_devres.o
 libphy-$(CONFIG_SWPHY)		+= swphy.o
 libphy-$(CONFIG_LED_TRIGGER_PHY)	+= phy_led_triggers.o

drivers/net/phy/mdio-debugfs.c

@@ -0,0 +1,196 @@
+#include <linux/module.h>
+#include <linux/debugfs.h>
+#include <linux/phy.h>
+#include <linux/poll.h>
+
+/*
+ * TODO: May need locking implementation to avoid being susceptible to file
+ * descriptor sharing concurrency issues
+ */
+struct mdio_debug {
+	wait_queue_head_t queue;
+	int value;
+};
+
+static int mdio_debug_open(struct inode *inode, struct file *file)
+{
+	struct mii_bus *bus = file->f_inode->i_private;
+	struct mdio_debug *data;
+	int err;
+
+	data = kzalloc(sizeof *data, GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	err = mutex_lock_interruptible(&bus->mdio_lock);
+	if (err) {
+		kfree(data);
+		return err;
+	}
+	dev_dbg(&bus->dev, "MDIO locked for user space program.\n");
+
+	init_waitqueue_head(&data->queue);
+	data->value = -1;
+	file->private_data = data;
+
+	return 0;
+}
+
+static int mdio_debug_release(struct inode *inode, struct file *file)
+{
+	struct mii_bus *bus = file->f_inode->i_private;
+	struct mdio_debug *data = file->private_data;
+
+	file->private_data = NULL;
+
+	mutex_unlock(&bus->mdio_lock);
+	dev_dbg(&bus->dev, "MDIO unlocked.\n");
+
+	kfree(data);
+
+	return 0;
+}
+
+static ssize_t mdio_debug_write(struct file *file, const char __user *buffer, size_t size, loff_t *off)
+{
+	struct mii_bus *bus = file->f_inode->i_private;
+	struct mdio_debug *data = file->private_data;
+	char str[64] = {};
+	char *s = str;
+	char *token;
+	int addr;
+	int offset;
+	int value;
+	int ret;
+
+	if (data->value != -1)
+		return -EWOULDBLOCK;
+
+	if (size > sizeof str - 1)
+		return -EINVAL;
+
+	ret = copy_from_user(str, buffer, size);
+	if (ret)
+		return -EFAULT;
+
+	if (str[size-1] == '\n')
+		str[size-1] = '\0';
+
+	token = strsep(&s, ":");
+	if (!token)
+		return -EINVAL;
+	ret = kstrtoint(token, 16, &addr);
+	if (ret)
+		return ret;
+
+	token = strsep(&s, ":");
+	if (!token)
+		return -EINVAL;
+	ret = kstrtoint(token, 16, &offset);
+	if (ret)
+		return ret;
+
+	token = strsep(&s, ":");
+
+	if (token) {
+		ret = kstrtoint(token, 16, &value);
+		if (ret)
+			return ret;
+
+		ret = __mdiobus_write(bus, addr, offset, value);
+		if (ret)
+			return ret;
+
+		dev_dbg(&bus->dev, "write: addr=0x%.2x offset=0x%.2x value=%.4x\n",
+			addr, offset, value);
+	} else {
+		value = __mdiobus_read(bus, addr, offset);
+		if (value < 0)
+			return value;
+
+		dev_dbg(&bus->dev, "read: addr=0x%.2x offset=0x%.2x value=%.4x\n",
+			addr, offset, value);
+
+		data->value = value;
+		wake_up_all(&data->queue);
+	}
+
+	return size;
+}
+
+static ssize_t mdio_debug_read(struct file *file, char __user *buffer, size_t size, loff_t *off)
+{
+	struct mdio_debug *data = file->private_data;
+	char str[6];
+	int ret;
+	ssize_t rsize;
+
+	if (data->value == -1)
+		return -EWOULDBLOCK;
+
+	rsize = snprintf(str, sizeof str, "%04x\n", data->value);
+	if (rsize > size)
+		return -EINVAL;
+
+	ret = copy_to_user(buffer, str, rsize);
+	if (ret)
+		return -EFAULT;
+
+	data->value = -1;
+	wake_up_all(&data->queue);
+
+	return rsize;
+}
+
+static unsigned int mdio_debug_poll(struct file *file, poll_table *wait)
+{
+	struct mdio_debug *data = file->private_data;
+
+	poll_wait(file, &data->queue, wait);
+
+	return data->value == -1 ? POLLOUT : POLLIN;
+}
+
+struct file_operations mdio_debug_fops = {
+	.owner = THIS_MODULE,
+	.open = mdio_debug_open,
+	.release = mdio_debug_release,
+	.write = mdio_debug_write,
+	.read = mdio_debug_read,
+	.poll = mdio_debug_poll,
+};
+
+/*
+ * TODO: This implementation doesn't support module load/unload and has no
+ * error checking.
+ */
+
+static struct dentry *mdio_debugfs_dentry;
+
+void mdio_debugfs_add(struct mii_bus *bus)
+{
+	bus->debugfs_dentry = debugfs_create_dir(dev_name(&bus->dev), mdio_debugfs_dentry);
+	debugfs_create_file("control", 0600, bus->debugfs_dentry, bus, &mdio_debug_fops);
+}
+EXPORT_SYMBOL_GPL(mdio_debugfs_add);
+
+void mdio_debugfs_remove(struct mii_bus *bus)
+{
+	debugfs_remove(bus->debugfs_dentry);
+	bus->debugfs_dentry = NULL;
+}
+EXPORT_SYMBOL_GPL(mdio_debugfs_remove);
+
+int __init mdio_debugfs_init(void)
+{
+	mdio_debugfs_dentry = debugfs_create_dir("mdio", NULL);
+
+	return PTR_ERR_OR_ZERO(mdio_debugfs_dentry);
+}
+module_init(mdio_debugfs_init);
+
+void __exit mdio_debugfs_exit(void)
+{
+	debugfs_remove(mdio_debugfs_dentry);
+}
+module_exit(mdio_debugfs_exit);

drivers/net/phy/mdio-debugfs.h

@@ -0,0 +1,2 @@
+void mdio_debugfs_add(struct mii_bus *bus);
+void mdio_debugfs_remove(struct mii_bus *bus);

drivers/net/phy/mdio_bus.c

@@ -39,6 +39,7 @@
 #include <trace/events/mdio.h>
 
 #include "mdio-boardinfo.h"
+#include "mdio-debugfs.h"
 
 static int mdiobus_register_gpiod(struct mdio_device *mdiodev)
 {
@@ -581,6 +582,8 @@ int __mdiobus_register(struct mii_bus *bus, struct module *owner)
 
 	mdiobus_setup_mdiodev_from_board_info(bus, mdiobus_create_device);
 
+	mdio_debugfs_add(bus);
+
 	bus->state = MDIOBUS_REGISTERED;
 	pr_info("%s: probed\n", bus->name);
 	return 0;
@@ -612,6 +615,8 @@ void mdiobus_unregister(struct mii_bus *bus)
 	BUG_ON(bus->state != MDIOBUS_REGISTERED);
 	bus->state = MDIOBUS_UNREGISTERED;
 
+	mdio_debugfs_remove(bus);
+
 	for (i = 0; i < PHY_MAX_ADDR; i++) {
 		mdiodev = bus->mdio_map[i];
 		if (!mdiodev)

include/linux/phy.h

@@ -371,6 +371,9 @@ struct mii_bus {
 
 	/** @shared: shared state across different PHYs */
 	struct phy_package_shared *shared[PHY_MAX_ADDR];
+
+	/* debugfs file for MDIO access */
+	struct dentry *debugfs_dentry;
 };
 #define to_mii_bus(d) container_of(d, struct mii_bus, dev)