can: peak_usb: export PCAN CAN channel ID as sysfs device attribute

This patch exports the CAN channel ID as a sysfs attribute. The CAN
channel ID is a user-configurable u8/u32 identifier that can be set
individually for each CAN interface of a PEAK USB device.

Exporting the channel ID as a sysfs attribute allows users to easily read
the ID and to write udev rules that can match against the ID. This is
especially useful for PEAK USB devices that do not export a serial
number at SUB level.
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Lukas Magel <lukas.magel@posteo.net>
Link: https://lore.kernel.org/all/20230116200932.157769-7-lukas.magel@posteo.netSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/ABI/testing/sysfs-class-net-peak_usb

+
+What:		/sys/class/net/<iface>/peak_usb/can_channel_id
+Date:		November 2022
+KernelVersion:	6.2
+Contact:	Stephane Grosjean <s.grosjean@peak-system.com>
+Description:
+		PEAK PCAN-USB devices support user-configurable CAN channel
+		identifiers. Contrary to a USB serial number, these identifiers
+		are writable and can be set per CAN interface. This means that
+		if a USB device exports multiple CAN interfaces, each of them
+		can be assigned a unique channel ID.
+		This attribute provides read-only access to the currently
+		configured value of the channel identifier. Depending on the
+		device type, the identifier has a length of 8 or 32 bit. The
+		value read from this attribute is always an 8 digit 32 bit
+		hexadecimal value in big endian format. If the device only
+		supports an 8 bit identifier, the upper 24 bit of the value are
+		set to zero.
+

drivers/net/can/usb/peak_usb/pcan_usb_core.c

@@ -15,6 +15,8 @@
 #include <linux/netdevice.h>
 #include <linux/usb.h>
 #include <linux/ethtool.h>
+#include <linux/sysfs.h>
+#include <linux/device.h>
 
 #include <linux/can.h>
 #include <linux/can/dev.h>
@@ -53,6 +55,26 @@ static const struct usb_device_id peak_usb_table[] = {
 
 MODULE_DEVICE_TABLE(usb, peak_usb_table);
 
+static ssize_t can_channel_id_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct net_device *netdev = to_net_dev(dev);
+	struct peak_usb_device *peak_dev = netdev_priv(netdev);
+
+	return sysfs_emit(buf, "%08X\n", peak_dev->can_channel_id);
+}
+static DEVICE_ATTR_RO(can_channel_id);
+
+/* mutable to avoid cast in attribute_group */
+static struct attribute *peak_usb_sysfs_attrs[] = {
+	&dev_attr_can_channel_id.attr,
+	NULL,
+};
+
+static const struct attribute_group peak_usb_sysfs_group = {
+	.name	= "peak_usb",
+	.attrs	= peak_usb_sysfs_attrs,
+};
+
 /*
  * dump memory
  */
@@ -961,6 +983,9 @@ static int peak_usb_create_dev(const struct peak_usb_adapter *peak_usb_adapter,
 	/* add ethtool support */
 	netdev->ethtool_ops = peak_usb_adapter->ethtool_ops;
 
+	/* register peak_usb sysfs files */
+	netdev->sysfs_groups[0] = &peak_usb_sysfs_group;
+
 	init_usb_anchor(&dev->rx_submitted);
 
 	init_usb_anchor(&dev->tx_submitted);