can: dev: provide optional GPIO based termination support

For CAN buses to work, a termination resistor has to be present at both
ends of the bus. This resistor is usually 120 Ohms, other values may be
required for special bus topologies.

This patch adds support for a generic GPIO based CAN termination. The
resistor value has to be specified via device tree, and it can only be
attached to or detached from the bus. By default the termination is not
active.

Link: https://lore.kernel.org/r/20210818071232.20585-4-o.rempel@pengutronix.de
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/drivers/net/can/dev/dev.c b/drivers/net/can/dev/dev.c
index 311d8564d6111d5bc8e7b3201714e07b40ea5ee4..e3d840b81357d6e529b2131464d1e446f5494c5e 100644 (file)
--- a/drivers/net/can/dev/dev.c
+++ b/drivers/net/can/dev/dev.c
@@ -15,6 +15,7 @@
 #include <linux/can/dev.h>
 #include <linux/can/skb.h>
 #include <linux/can/led.h>
+#include <linux/gpio/consumer.h>
 #include <linux/of.h>
 
 #define MOD_DESC "CAN device driver interface"
@@ -400,10 +401,69 @@ void close_candev(struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(close_candev);
 
+static int can_set_termination(struct net_device *ndev, u16 term)
+{
+       struct can_priv *priv = netdev_priv(ndev);
+       int set;
+
+       if (term == priv->termination_gpio_ohms[CAN_TERMINATION_GPIO_ENABLED])
+               set = 1;
+       else
+               set = 0;
+
+       gpiod_set_value(priv->termination_gpio, set);
+
+       return 0;
+}
+
+static int can_get_termination(struct net_device *ndev)
+{
+       struct can_priv *priv = netdev_priv(ndev);
+       struct device *dev = ndev->dev.parent;
+       struct gpio_desc *gpio;
+       u32 term;
+       int ret;
+
+       /* Disabling termination by default is the safe choice: Else if many
+        * bus participants enable it, no communication is possible at all.
+        */
+       gpio = devm_gpiod_get_optional(dev, "termination", GPIOD_OUT_LOW);
+       if (IS_ERR(gpio))
+               return dev_err_probe(dev, PTR_ERR(gpio),
+                                    "Cannot get termination-gpios\n");
+
+       if (!gpio)
+               return 0;
+
+       ret = device_property_read_u32(dev, "termination-ohms", &term);
+       if (ret) {
+               netdev_err(ndev, "Cannot get termination-ohms: %pe\n",
+                          ERR_PTR(ret));
+               return ret;
+       }
+
+       if (term > U16_MAX) {
+               netdev_err(ndev, "Invalid termination-ohms value (%u > %u)\n",
+                          term, U16_MAX);
+               return -EINVAL;
+       }
+
+       priv->termination_const_cnt = ARRAY_SIZE(priv->termination_gpio_ohms);
+       priv->termination_const = priv->termination_gpio_ohms;
+       priv->termination_gpio = gpio;
+       priv->termination_gpio_ohms[CAN_TERMINATION_GPIO_DISABLED] =
+               CAN_TERMINATION_DISABLED;
+       priv->termination_gpio_ohms[CAN_TERMINATION_GPIO_ENABLED] = term;
+       priv->do_set_termination = can_set_termination;
+
+       return 0;
+}
+
 /* Register the CAN network device */
 int register_candev(struct net_device *dev)
 {
        struct can_priv *priv = netdev_priv(dev);
+       int err;
 
        /* Ensure termination_const, termination_const_cnt and
         * do_set_termination consistency. All must be either set or
@@ -419,6 +479,12 @@ int register_candev(struct net_device *dev)
        if (!priv->data_bitrate_const != !priv->data_bitrate_const_cnt)
                return -EINVAL;
 
+       if (!priv->termination_const) {
+               err = can_get_termination(dev);
+               if (err)
+                       return err;
+       }
+
        dev->rtnl_link_ops = &can_link_ops;
        netif_carrier_off(dev);
 

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 27b275e463da3079059806e8638aece0220001d7..2413253e54c704546b2aa051efc611ba01ad1511 100644 (file)
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -32,6 +32,12 @@ enum can_mode {
        CAN_MODE_SLEEP
 };
 
+enum can_termination_gpio {
+       CAN_TERMINATION_GPIO_DISABLED = 0,
+       CAN_TERMINATION_GPIO_ENABLED,
+       CAN_TERMINATION_GPIO_MAX,
+};
+
 /*
  * CAN common private data
  */
@@ -55,6 +61,8 @@ struct can_priv {
        unsigned int termination_const_cnt;
        const u16 *termination_const;
        u16 termination;
+       struct gpio_desc *termination_gpio;
+       u16 termination_gpio_ohms[CAN_TERMINATION_GPIO_MAX];
 
        enum can_state state;