net: sfp: add debugfs support

Add debugfs support to SFP so that the internal state of the SFP state
machines and hardware signal state can be viewed from userspace, rather
than having to compile a debug kernel to view state transitions in the
kernel log.  The 'state' output looks like:

Module state: empty
Module probe attempts: 0 0
Device state: up
Main state: down
Fault recovery remaining retries: 5
PHY probe remaining retries: 12
moddef0: 0
rx_los: 1
tx_fault: 1
tx_disable: 1

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1kyYRe-0004kN-3F@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/sfp.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/acpi.h>
 #include <linux/ctype.h>
+#include <linux/debugfs.h>
 #include <linux/delay.h>
 #include <linux/gpio/consumer.h>
 #include <linux/hwmon.h>
@@ -258,6 +259,9 @@ struct sfp {
 	char *hwmon_name;
 #endif
 
+#if IS_ENABLED(CONFIG_DEBUG_FS)
+	struct dentry *debugfs_dir;
+#endif
 };
 
 static bool sff_module_supported(const struct sfp_eeprom_id *id)
@@ -1390,6 +1394,54 @@ static void sfp_module_tx_enable(struct sfp *sfp)
 	sfp_set_state(sfp, sfp->state);
 }
 
+#if IS_ENABLED(CONFIG_DEBUG_FS)
+static int sfp_debug_state_show(struct seq_file *s, void *data)
+{
+	struct sfp *sfp = s->private;
+
+	seq_printf(s, "Module state: %s\n",
+		   mod_state_to_str(sfp->sm_mod_state));
+	seq_printf(s, "Module probe attempts: %d %d\n",
+		   R_PROBE_RETRY_INIT - sfp->sm_mod_tries_init,
+		   R_PROBE_RETRY_SLOW - sfp->sm_mod_tries);
+	seq_printf(s, "Device state: %s\n",
+		   dev_state_to_str(sfp->sm_dev_state));
+	seq_printf(s, "Main state: %s\n",
+		   sm_state_to_str(sfp->sm_state));
+	seq_printf(s, "Fault recovery remaining retries: %d\n",
+		   sfp->sm_fault_retries);
+	seq_printf(s, "PHY probe remaining retries: %d\n",
+		   sfp->sm_phy_retries);
+	seq_printf(s, "moddef0: %d\n", !!(sfp->state & SFP_F_PRESENT));
+	seq_printf(s, "rx_los: %d\n", !!(sfp->state & SFP_F_LOS));
+	seq_printf(s, "tx_fault: %d\n", !!(sfp->state & SFP_F_TX_FAULT));
+	seq_printf(s, "tx_disable: %d\n", !!(sfp->state & SFP_F_TX_DISABLE));
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(sfp_debug_state);
+
+static void sfp_debugfs_init(struct sfp *sfp)
+{
+	sfp->debugfs_dir = debugfs_create_dir(dev_name(sfp->dev), NULL);
+
+	debugfs_create_file("state", 0600, sfp->debugfs_dir, sfp,
+			    &sfp_debug_state_fops);
+}
+
+static void sfp_debugfs_exit(struct sfp *sfp)
+{
+	debugfs_remove_recursive(sfp->debugfs_dir);
+}
+#else
+static void sfp_debugfs_init(struct sfp *sfp)
+{
+}
+
+static void sfp_debugfs_exit(struct sfp *sfp)
+{
+}
+#endif
+
 static void sfp_module_tx_fault_reset(struct sfp *sfp)
 {
 	unsigned int state = sfp->state;
@@ -2491,13 +2543,16 @@ static int sfp_probe(struct platform_device *pdev)
 	if (!sfp->sfp_bus)
 		return -ENOMEM;
 
+	sfp_debugfs_init(sfp);
+
 	return 0;
 }
 
 static int sfp_remove(struct platform_device *pdev)
 {
 	struct sfp *sfp = platform_get_drvdata(pdev);
 
+	sfp_debugfs_exit(sfp);
 	sfp_unregister_socket(sfp->sfp_bus);
 
 	rtnl_lock();