[PATCH v6 2/2] i2c: i2c-gpio: Enhance driver for buses with shared SCL

[Markus Stockhausen]

Some lower end hardware (especially Realtek based switches) are
designed with multiple I2C buses that share a single clock line.
E.g. the D-Link DGS-1250-28X realizes 4 I2C SFP busses with 5 gpios.

Enhance the i2c-gpio driver so it can handle such hardware designs.

- Detect GPIOs that are used by multiple I2C buses in the dts by
  using a unique identifier for each managed SCL.

- The first probing instance allocates and requests the shared SCL
  GPIO plus an associated rt_mutex. Subsequent instances detect the
  existing entry via the identifier and increment a reference count
  to reuse the descriptor.

- All data transfers are serialized via custom lock_ops that handle
  both the standard adapter bus lock and the shared SCL mutex. This
  ensures mutual exclusion across adapters sharing the clock line.

This patch was successfully tested on Linksys LGS310C that has two
SFP slots with two GPIO based I2C buses that share a single SCL.
Test environment: OpenWrt snapshot ported to kernel 6.19.14
including CONFIG_GPIO_SHARED=y and CONFIG_GPIO_SHARED_PROXY=y.

Remark: This shared SCL detection works only for dts based systems.
On legacy systems without dts each gpio pin will be handled
indidivudally (as before).

Signed-off-by: Markus Stockhausen <markus.stockhausen@xxxxxx>
Tested-by: Sander Vanheule <sander@xxxxxxxxxxxxx>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>
Reviewed-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/i2c/busses/i2c-gpio.c | 223 +++++++++++++++++++++++++++++++---
 1 file changed, 209 insertions(+), 14 deletions(-)

diff --git a/drivers/i2c/busses/i2c-gpio.c b/drivers/i2c/busses/i2c-gpio.c
index f4355b17bfbf..bb112f529dcd 100644
--- a/drivers/i2c/busses/i2c-gpio.c
+++ b/drivers/i2c/busses/i2c-gpio.c
@@ -18,9 +18,23 @@
 #include <linux/property.h>
 #include <linux/slab.h>
 
+static LIST_HEAD(i2c_gpio_scl_list);
+static DEFINE_MUTEX(i2c_gpio_scl_list_lock);
+static struct lock_class_key i2c_gpio_scl_lock_key;
+
+struct i2c_gpio_scl_data {
+	struct fwnode_handle *fw_node;
+	u32 fw_pin;
+	u32 fw_flags;
+	struct gpio_desc *gpio;
+	struct rt_mutex lock;
+	refcount_t ref;
+	struct list_head list;
+};
+
 struct i2c_gpio_private_data {
 	struct gpio_desc *sda;
-	struct gpio_desc *scl;
+	struct i2c_gpio_scl_data *scl;
 	struct i2c_adapter adap;
 	struct i2c_algo_bit_data bit_data;
 	struct i2c_gpio_platform_data pdata;
@@ -31,6 +45,11 @@ struct i2c_gpio_private_data {
 #endif
 };
 
+static inline struct i2c_gpio_private_data *adap_to_priv(struct i2c_adapter *adap)
+{
+	return container_of(adap, struct i2c_gpio_private_data, adap);
+}
+
 /*
  * Toggle SDA by changing the output value of the pin. This is only
  * valid for pins configured as open drain (i.e. setting the value
@@ -53,7 +72,7 @@ static void i2c_gpio_setscl_val(void *data, int state)
 {
 	struct i2c_gpio_private_data *priv = data;
 
-	gpiod_set_value_cansleep(priv->scl, state);
+	gpiod_set_value_cansleep(priv->scl->gpio, state);
 }
 
 static int i2c_gpio_getsda(void *data)
@@ -67,9 +86,41 @@ static int i2c_gpio_getscl(void *data)
 {
 	struct i2c_gpio_private_data *priv = data;
 
-	return gpiod_get_value_cansleep(priv->scl);
+	return gpiod_get_value_cansleep(priv->scl->gpio);
+}
+
+static void i2c_gpio_lock_bus(struct i2c_adapter *adap, unsigned int flags)
+{
+	/* Take care about adapter lock. See i2c_adapter_lock_bus() and others. */
+	rt_mutex_lock_nested(&adap->bus_lock, i2c_adapter_depth(adap));
+	rt_mutex_lock(&adap_to_priv(adap)->scl->lock);
+}
+
+static int i2c_gpio_trylock_bus(struct i2c_adapter *adap, unsigned int flags)
+{
+	if (!rt_mutex_trylock(&adap->bus_lock))
+		return 0;
+
+	if (!rt_mutex_trylock(&adap_to_priv(adap)->scl->lock)) {
+		rt_mutex_unlock(&adap->bus_lock);
+		return 0;
+	}
+
+	return 1;
 }
 
+static void i2c_gpio_unlock_bus(struct i2c_adapter *adap, unsigned int flags)
+{
+	rt_mutex_unlock(&adap_to_priv(adap)->scl->lock);
+	rt_mutex_unlock(&adap->bus_lock);
+}
+
+static const struct i2c_lock_operations i2c_gpio_lock_ops = {
+	.lock_bus = i2c_gpio_lock_bus,
+	.trylock_bus = i2c_gpio_trylock_bus,
+	.unlock_bus = i2c_gpio_unlock_bus,
+};
+
 #ifdef CONFIG_I2C_GPIO_FAULT_INJECTOR
 
 #define setsda(bd, val)	((bd)->setsda((bd)->data, val))
@@ -165,14 +216,14 @@ DEFINE_DEBUGFS_ATTRIBUTE(fops_incomplete_write_byte, NULL, fops_incomplete_write
 static int i2c_gpio_fi_act_on_scl_irq(struct i2c_gpio_private_data *priv,
 				       irqreturn_t handler(int, void*))
 {
-	int ret, irq = gpiod_to_irq(priv->scl);
+	int ret, irq = gpiod_to_irq(priv->scl->gpio);
 
 	if (irq < 0)
 		return irq;
 
 	i2c_lock_bus(&priv->adap, I2C_LOCK_ROOT_ADAPTER);
 
-	ret = gpiod_direction_input(priv->scl);
+	ret = gpiod_direction_input(priv->scl->gpio);
 	if (ret)
 		goto unlock;
 
@@ -187,7 +238,7 @@ static int i2c_gpio_fi_act_on_scl_irq(struct i2c_gpio_private_data *priv,
 
 	free_irq(irq, priv);
  output:
-	ret = gpiod_direction_output(priv->scl, 1) ?: ret;
+	ret = gpiod_direction_output(priv->scl->gpio, 1) ?: ret;
  unlock:
 	i2c_unlock_bus(&priv->adap, I2C_LOCK_ROOT_ADAPTER);
 
@@ -308,13 +359,17 @@ static struct gpio_desc *i2c_gpio_get_desc(struct device *dev,
 	struct gpio_desc *retdesc;
 	int ret;
 
-	retdesc = devm_gpiod_get(dev, con_id, gflags);
+	/*
+	 * Don't use resource-managed functions. SCL may be shared across adapters and has
+	 * its own lifetime management. SDA uses the same path for consistency.
+	 */
+	retdesc = gpiod_get(dev, con_id, gflags);
 	if (!IS_ERR(retdesc)) {
 		dev_dbg(dev, "got GPIO from name %s\n", con_id);
 		return retdesc;
 	}
 
-	retdesc = devm_gpiod_get_index(dev, NULL, index, gflags);
+	retdesc = gpiod_get_index(dev, NULL, index, gflags);
 	if (!IS_ERR(retdesc)) {
 		dev_dbg(dev, "got GPIO from index %u\n", index);
 		return retdesc;
@@ -336,6 +391,132 @@ static struct gpio_desc *i2c_gpio_get_desc(struct device *dev,
 	return retdesc;
 }
 
+static struct i2c_gpio_scl_data *i2c_gpio_create_scl(struct device *dev)
+{
+	struct fwnode_handle *fwnode = dev_fwnode(dev);
+	struct fwnode_reference_args args;
+	struct i2c_gpio_scl_data *scl;
+	int ret;
+
+	if (fwnode) {
+		ret = fwnode_property_get_reference_args(fwnode, "scl-gpios",
+							 "#gpio-cells", 0, 0, &args);
+		if (ret)
+			/* try the ancient way */
+			ret = fwnode_property_get_reference_args(fwnode, "gpios",
+								 "#gpio-cells", 0, 1, &args);
+		if (ret)
+			return ERR_PTR(ret);
+
+		if (args.nargs < 1) {
+			fwnode_handle_put(args.fwnode);
+			return ERR_PTR(-EINVAL);
+		}
+	} else {
+		/* legacy system without device tree */
+		args.fwnode = NULL;
+		args.args[0] = 0;
+		args.nargs = 0;
+	}
+
+	scl = kzalloc(sizeof(*scl), GFP_KERNEL);
+	if (!scl) {
+		fwnode_handle_put(args.fwnode);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	scl->fw_node = args.fwnode;
+	scl->fw_pin = args.args[0];
+	scl->fw_flags = (args.nargs >= 2) ? args.args[1] : 0;
+
+	rt_mutex_init(&scl->lock);
+	lockdep_set_class(&scl->lock, &i2c_gpio_scl_lock_key);
+	refcount_set(&scl->ref, 1);
+
+	return scl;
+}
+
+static void i2c_gpio_free_scl(struct i2c_gpio_scl_data *scl)
+{
+	if (scl->fw_node)
+		fwnode_handle_put(scl->fw_node);
+	kfree(scl);
+}
+
+static bool i2c_gpio_scl_matches(struct i2c_gpio_scl_data *a, struct i2c_gpio_scl_data *b)
+{
+	/* legacy systems do not support shared SCL */
+	if (!a->fw_node || !b->fw_node)
+		return false;
+
+	return a->fw_node == b->fw_node && a->fw_pin == b->fw_pin && a->fw_flags == b->fw_flags;
+}
+
+/*
+ * Look up an existing or create a new shared SCL structure described by the device's fwnode.
+ * Optimistic setup sequence always creates and tries to add a new entry to the list. This uses
+ * minimum locking and afterwards requests the GPIO without a lock held. Concurrent probes for
+ * the same SCL pin see the entry and do not race into a second gpiod_get(). Until everything
+ * is setup they terminate with -EPROBE_DEFER.
+ */
+static struct i2c_gpio_scl_data *i2c_gpio_lookup_scl(struct device *dev, enum gpiod_flags gflags)
+{
+	struct i2c_gpio_scl_data *scl, *new_scl;
+	struct gpio_desc *gpio;
+
+	new_scl = i2c_gpio_create_scl(dev);
+	if (IS_ERR(new_scl))
+		return new_scl;
+
+	scoped_guard(mutex, &i2c_gpio_scl_list_lock) {
+		list_for_each_entry(scl, &i2c_gpio_scl_list, list) {
+			if (!i2c_gpio_scl_matches(scl, new_scl))
+				continue;
+
+			i2c_gpio_free_scl(new_scl);
+			if (!scl->gpio)
+				return ERR_PTR(-EPROBE_DEFER);
+
+			refcount_inc(&scl->ref);
+			if (scl->fw_node)
+				dev_dbg(dev, "reusing shared SCL (%pfwP, pin %u)\n",
+					scl->fw_node, scl->fw_pin);
+
+			return scl;
+		}
+		list_add(&new_scl->list, &i2c_gpio_scl_list);
+	}
+
+	gpio = i2c_gpio_get_desc(dev, "scl", 1, gflags);
+	if (IS_ERR(gpio)) {
+		scoped_guard(mutex, &i2c_gpio_scl_list_lock)
+			list_del(&new_scl->list);
+		i2c_gpio_free_scl(new_scl);
+
+		return ERR_CAST(gpio);
+	}
+
+	scoped_guard(mutex, &i2c_gpio_scl_list_lock)
+		new_scl->gpio = gpio;
+
+	if (new_scl->fw_node)
+		dev_dbg(dev, "registered shared SCL (%pfwP, pin %u)\n",
+			new_scl->fw_node, new_scl->fw_pin);
+
+	return new_scl;
+}
+
+static void i2c_gpio_cleanup_scl(struct i2c_gpio_scl_data *scl)
+{
+	if (!refcount_dec_and_mutex_lock(&scl->ref, &i2c_gpio_scl_list_lock))
+		return;
+
+	list_del(&scl->list);
+	mutex_unlock(&i2c_gpio_scl_list_lock);
+	gpiod_put(scl->gpio);
+	i2c_gpio_free_scl(scl);
+}
+
 static int i2c_gpio_probe(struct platform_device *pdev)
 {
 	struct i2c_gpio_private_data *priv;
@@ -386,15 +567,18 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 		gflags = GPIOD_OUT_HIGH;
 	else
 		gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;
-	priv->scl = i2c_gpio_get_desc(dev, "scl", 1, gflags);
-	if (IS_ERR(priv->scl))
-		return PTR_ERR(priv->scl);
+	priv->scl = i2c_gpio_lookup_scl(dev, gflags);
+	if (IS_ERR(priv->scl)) {
+		ret = PTR_ERR(priv->scl);
+		goto err_cleanup_sda;
+	}
 
-	if (gpiod_cansleep(priv->sda) || gpiod_cansleep(priv->scl))
+	if (gpiod_cansleep(priv->sda) || gpiod_cansleep(priv->scl->gpio))
 		dev_warn(dev, "Slow GPIO pins might wreak havoc into I2C/SMBus bus timing");
 	else
 		bit_data->can_do_atomic = true;
 
+	bit_data->skip_bit_test = true;
 	bit_data->setsda = i2c_gpio_setsda_val;
 	bit_data->setscl = i2c_gpio_setscl_val;
 
@@ -423,6 +607,8 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	else
 		snprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);
 
+	/* Always use shared SCL aware locking */
+	adap->lock_ops = &i2c_gpio_lock_ops;
 	adap->algo_data = bit_data;
 	adap->class = I2C_CLASS_HWMON;
 	adap->dev.parent = dev;
@@ -431,7 +617,7 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	adap->nr = pdev->id;
 	ret = i2c_bit_add_numbered_bus(adap);
 	if (ret)
-		return ret;
+		goto err_cleanup_scl;
 
 	platform_set_drvdata(pdev, priv);
 
@@ -441,13 +627,20 @@ static int i2c_gpio_probe(struct platform_device *pdev)
 	 * from the descriptor, then provide that instead.
 	 */
 	dev_info(dev, "using lines %u (SDA) and %u (SCL%s)\n",
-		 desc_to_gpio(priv->sda), desc_to_gpio(priv->scl),
+		 desc_to_gpio(priv->sda), desc_to_gpio(priv->scl->gpio),
 		 pdata->scl_is_output_only
 		 ? ", no clock stretching" : "");
 
 	i2c_gpio_fault_injector_init(pdev);
 
 	return 0;
+
+err_cleanup_scl:
+	i2c_gpio_cleanup_scl(priv->scl);
+err_cleanup_sda:
+	gpiod_put(priv->sda);
+
+	return ret;
 }
 
 static void i2c_gpio_remove(struct platform_device *pdev)
@@ -459,6 +652,8 @@ static void i2c_gpio_remove(struct platform_device *pdev)
 	adap = &priv->adap;
 
 	i2c_del_adapter(adap);
+	i2c_gpio_cleanup_scl(priv->scl);
+	gpiod_put(priv->sda);
 }
 
 static const struct of_device_id i2c_gpio_dt_ids[] = {
-- 
2.54.0