@@ -47,6 +47,16 @@ config MCTP_TRANSPORT_I3C
A MCTP protocol network device is created for each I3C bus
having a "mctp-controller" devicetree property.
+config MCTP_TRANSPORT_USB
+ tristate "MCTP USB transport"
+ depends on USB
+ help
+ Provides a driver to access MCTP devices over USB transport,
+ defined by DMTF specification DSP0283.
+
+ MCTP-over-USB interfaces are peer-to-peer, so each interface
+ represents a physical connection to one remote MCTP endpoint.
+
endmenu
endif
@@ -1,3 +1,4 @@
obj-$(CONFIG_MCTP_SERIAL) += mctp-serial.o
obj-$(CONFIG_MCTP_TRANSPORT_I2C) += mctp-i2c.o
obj-$(CONFIG_MCTP_TRANSPORT_I3C) += mctp-i3c.o
+obj-$(CONFIG_MCTP_TRANSPORT_USB) += mctp-usb.o
new file mode 100644
@@ -0,0 +1,368 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * mctp-usb.c - MCTP-over-USB (DMTF DSP0283) transport binding driver.
+ *
+ * DSP0283 is available at:
+ * https://www.dmtf.org/sites/default/files/standards/documents/DSP0283_1.0.1.pdf
+ *
+ * Copyright (C) 2024-2025 Code Construct Pty Ltd
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/usb.h>
+#include <linux/usb/mctp-usb.h>
+
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+#include <net/pkt_sched.h>
+
+#include <uapi/linux/if_arp.h>
+
+struct mctp_usb {
+ struct usb_device *usbdev;
+ struct usb_interface *intf;
+
+ struct net_device *netdev;
+
+ __u8 ep_in;
+ __u8 ep_out;
+
+ struct urb *tx_urb;
+ struct urb *rx_urb;
+};
+
+static void mctp_usb_stat_tx_dropped(struct net_device *dev)
+{
+ struct pcpu_dstats *dstats = this_cpu_ptr(dev->dstats);
+
+ u64_stats_update_begin(&dstats->syncp);
+ u64_stats_inc(&dstats->tx_drops);
+ u64_stats_update_end(&dstats->syncp);
+}
+
+static void mctp_usb_stat_tx_done(struct net_device *dev, unsigned int len)
+{
+ struct pcpu_dstats *dstats = this_cpu_ptr(dev->dstats);
+
+ u64_stats_update_begin(&dstats->syncp);
+ u64_stats_inc(&dstats->tx_packets);
+ u64_stats_add(&dstats->tx_bytes, len);
+ u64_stats_update_end(&dstats->syncp);
+}
+
+static void mctp_usb_out_complete(struct urb *urb)
+{
+ struct sk_buff *skb = urb->context;
+ struct net_device *netdev = skb->dev;
+ int status;
+
+ status = urb->status;
+
+ switch (status) {
+ case -ENOENT:
+ case -ECONNRESET:
+ case -ESHUTDOWN:
+ case -EPROTO:
+ mctp_usb_stat_tx_dropped(netdev);
+ break;
+ case 0:
+ mctp_usb_stat_tx_done(netdev, skb->len);
+ netif_wake_queue(netdev);
+ consume_skb(skb);
+ return;
+ default:
+ netdev_dbg(netdev, "unexpected tx urb status: %d\n", status);
+ mctp_usb_stat_tx_dropped(netdev);
+ }
+
+ kfree_skb(skb);
+}
+
+static netdev_tx_t mctp_usb_start_xmit(struct sk_buff *skb,
+ struct net_device *dev)
+{
+ struct mctp_usb *mctp_usb = netdev_priv(dev);
+ struct mctp_usb_hdr *hdr;
+ unsigned int plen;
+ struct urb *urb;
+ int rc;
+
+ plen = skb->len;
+
+ if (plen + sizeof(*hdr) > MCTP_USB_XFER_SIZE)
+ goto err_drop;
+
+ hdr = skb_push(skb, sizeof(*hdr));
+ if (!hdr)
+ goto err_drop;
+
+ hdr->id = cpu_to_be16(MCTP_USB_DMTF_ID);
+ hdr->rsvd = 0;
+ hdr->len = plen + sizeof(*hdr);
+
+ urb = mctp_usb->tx_urb;
+
+ usb_fill_bulk_urb(urb, mctp_usb->usbdev,
+ usb_sndbulkpipe(mctp_usb->usbdev, mctp_usb->ep_out),
+ skb->data, skb->len,
+ mctp_usb_out_complete, skb);
+
+ rc = usb_submit_urb(urb, GFP_ATOMIC);
+ if (rc)
+ goto err_drop;
+ else
+ netif_stop_queue(dev);
+
+ return NETDEV_TX_OK;
+
+err_drop:
+ mctp_usb_stat_tx_dropped(dev);
+ kfree_skb(skb);
+ return NETDEV_TX_OK;
+}
+
+static void mctp_usb_in_complete(struct urb *urb);
+
+static int mctp_usb_rx_queue(struct mctp_usb *mctp_usb, gfp_t gfp)
+{
+ struct sk_buff *skb;
+ int rc;
+
+ skb = __netdev_alloc_skb(mctp_usb->netdev, MCTP_USB_XFER_SIZE, gfp);
+ if (!skb)
+ return -ENOMEM;
+
+ usb_fill_bulk_urb(mctp_usb->rx_urb, mctp_usb->usbdev,
+ usb_rcvbulkpipe(mctp_usb->usbdev, mctp_usb->ep_in),
+ skb->data, MCTP_USB_XFER_SIZE,
+ mctp_usb_in_complete, skb);
+
+ rc = usb_submit_urb(mctp_usb->rx_urb, gfp);
+ if (rc) {
+ netdev_err(mctp_usb->netdev, "rx urb submit failure: %d\n", rc);
+ kfree_skb(skb);
+ }
+
+ return rc;
+}
+
+static void mctp_usb_in_complete(struct urb *urb)
+{
+ struct sk_buff *skb = urb->context;
+ struct net_device *netdev = skb->dev;
+ struct pcpu_dstats *dstats = this_cpu_ptr(netdev->dstats);
+ struct mctp_usb *mctp_usb = netdev_priv(netdev);
+ struct mctp_skb_cb *cb;
+ unsigned int len;
+ int status;
+
+ status = urb->status;
+
+ switch (status) {
+ case -ENOENT:
+ case -ECONNRESET:
+ case -ESHUTDOWN:
+ case -EPROTO:
+ kfree_skb(skb);
+ return;
+ case 0:
+ break;
+ default:
+ netdev_dbg(netdev, "unexpected rx urb status: %d\n", status);
+ kfree_skb(skb);
+ return;
+ }
+
+ len = urb->actual_length;
+ __skb_put(skb, len);
+
+ while (skb) {
+ struct sk_buff *skb2 = NULL;
+ struct mctp_usb_hdr *hdr;
+ u8 pkt_len; /* length of MCTP packet, no USB header */
+
+ hdr = skb_pull_data(skb, sizeof(*hdr));
+ if (!hdr)
+ break;
+
+ if (be16_to_cpu(hdr->id) != MCTP_USB_DMTF_ID) {
+ netdev_dbg(netdev, "rx: invalid id %04x\n",
+ be16_to_cpu(hdr->id));
+ break;
+ }
+
+ if (hdr->len <
+ sizeof(struct mctp_hdr) + sizeof(struct mctp_usb_hdr)) {
+ netdev_dbg(netdev, "rx: short packet (hdr) %d\n",
+ hdr->len);
+ break;
+ }
+
+ /* we know we have at least sizeof(struct mctp_usb_hdr) here */
+ pkt_len = hdr->len - sizeof(struct mctp_usb_hdr);
+ if (pkt_len > skb->len) {
+ netdev_dbg(netdev,
+ "rx: short packet (xfer) %d, actual %d\n",
+ hdr->len, skb->len);
+ break;
+ }
+
+ if (pkt_len < skb->len) {
+ /* more packets may follow - clone to a new
+ * skb to use on the next iteration
+ */
+ skb2 = skb_clone(skb, GFP_ATOMIC);
+ if (skb2) {
+ if (!skb_pull(skb2, pkt_len)) {
+ kfree_skb(skb2);
+ skb2 = NULL;
+ }
+ }
+ skb_trim(skb, pkt_len);
+ }
+
+ u64_stats_update_begin(&dstats->syncp);
+ u64_stats_inc(&dstats->rx_packets);
+ u64_stats_add(&dstats->rx_bytes, skb->len);
+ u64_stats_update_end(&dstats->syncp);
+
+ skb->protocol = htons(ETH_P_MCTP);
+ skb_reset_network_header(skb);
+ cb = __mctp_cb(skb);
+ cb->halen = 0;
+ netif_rx(skb);
+
+ skb = skb2;
+ }
+
+ if (skb)
+ kfree_skb(skb);
+
+ mctp_usb_rx_queue(mctp_usb, GFP_ATOMIC);
+}
+
+static int mctp_usb_open(struct net_device *dev)
+{
+ struct mctp_usb *mctp_usb = netdev_priv(dev);
+
+ return mctp_usb_rx_queue(mctp_usb, GFP_KERNEL);
+}
+
+static int mctp_usb_stop(struct net_device *dev)
+{
+ struct mctp_usb *mctp_usb = netdev_priv(dev);
+
+ netif_stop_queue(dev);
+ usb_kill_urb(mctp_usb->rx_urb);
+ usb_kill_urb(mctp_usb->tx_urb);
+
+ return 0;
+}
+
+static const struct net_device_ops mctp_usb_netdev_ops = {
+ .ndo_start_xmit = mctp_usb_start_xmit,
+ .ndo_open = mctp_usb_open,
+ .ndo_stop = mctp_usb_stop,
+};
+
+static void mctp_usb_netdev_setup(struct net_device *dev)
+{
+ dev->type = ARPHRD_MCTP;
+
+ dev->mtu = MCTP_USB_MTU_MIN;
+ dev->min_mtu = MCTP_USB_MTU_MIN;
+ dev->max_mtu = MCTP_USB_MTU_MAX;
+
+ dev->hard_header_len = sizeof(struct mctp_usb_hdr);
+ dev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
+ dev->addr_len = 0;
+ dev->flags = IFF_NOARP;
+ dev->netdev_ops = &mctp_usb_netdev_ops;
+ dev->needs_free_netdev = false;
+ dev->pcpu_stat_type = NETDEV_PCPU_STAT_DSTATS;
+}
+
+static int mctp_usb_probe(struct usb_interface *intf,
+ const struct usb_device_id *id)
+{
+ struct usb_endpoint_descriptor *ep_in, *ep_out;
+ struct usb_host_interface *iface_desc;
+ struct net_device *netdev;
+ struct mctp_usb *dev;
+ int rc;
+
+ /* only one alternate */
+ iface_desc = intf->cur_altsetting;
+
+ rc = usb_find_common_endpoints(iface_desc, &ep_in, &ep_out, NULL, NULL);
+ if (rc) {
+ dev_err(&intf->dev, "invalid endpoints on device?\n");
+ return rc;
+ }
+
+ netdev = alloc_netdev(sizeof(*dev), "mctpusb%d", NET_NAME_ENUM,
+ mctp_usb_netdev_setup);
+ if (!netdev)
+ return -ENOMEM;
+
+ SET_NETDEV_DEV(netdev, &intf->dev);
+ dev = netdev_priv(netdev);
+ dev->netdev = netdev;
+ dev->usbdev = usb_get_dev(interface_to_usbdev(intf));
+ dev->intf = intf;
+ usb_set_intfdata(intf, dev);
+
+ dev->ep_in = ep_in->bEndpointAddress;
+ dev->ep_out = ep_out->bEndpointAddress;
+
+ dev->tx_urb = usb_alloc_urb(0, GFP_KERNEL);
+ dev->rx_urb = usb_alloc_urb(0, GFP_KERNEL);
+ if (!dev->tx_urb || !dev->rx_urb) {
+ rc = -ENOMEM;
+ goto err_free_urbs;
+ }
+
+ rc = mctp_register_netdev(netdev, NULL, MCTP_PHYS_BINDING_USB);
+ if (rc)
+ goto err_free_urbs;
+
+ return 0;
+
+err_free_urbs:
+ usb_free_urb(dev->tx_urb);
+ usb_free_urb(dev->rx_urb);
+ free_netdev(netdev);
+ return rc;
+}
+
+static void mctp_usb_disconnect(struct usb_interface *intf)
+{
+ struct mctp_usb *dev = usb_get_intfdata(intf);
+
+ mctp_unregister_netdev(dev->netdev);
+ usb_free_urb(dev->tx_urb);
+ usb_free_urb(dev->rx_urb);
+ usb_put_dev(dev->usbdev);
+ free_netdev(dev->netdev);
+}
+
+static const struct usb_device_id mctp_usb_devices[] = {
+ { USB_INTERFACE_INFO(USB_CLASS_MCTP, 0x0, 0x1) },
+ { 0 },
+};
+
+MODULE_DEVICE_TABLE(usb, mctp_usb_devices);
+
+static struct usb_driver mctp_usb_driver = {
+ .name = "mctp-usb",
+ .id_table = mctp_usb_devices,
+ .probe = mctp_usb_probe,
+ .disconnect = mctp_usb_disconnect,
+};
+
+module_usb_driver(mctp_usb_driver)
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
+MODULE_DESCRIPTION("MCTP USB transport");
Add an implementation for DMTF DSP0283, which defines a MCTP-over-USB transport. As per that spec, we're restricted to full speed mode, requiring 512-byte transfers. Each MCTP-over-USB interface is a peer-to-peer link to a single MCTP endpoint, so no physical addressing is required (of course, that MCTP endpoint may then bridge to further MCTP endpoints). Consequently, interfaces will report with no lladdr data: # mctp link dev lo index 1 address 00:00:00:00:00:00 net 1 mtu 65536 up dev mctpusb0 index 6 address none net 1 mtu 68 up This is a simple initial implementation, with single rx & tx urbs, and no multi-packet tx transfers - although we do accept multi-packet rx from the device. Includes suggested fixes from Santosh Puranik <spuranik@nvidia.com>. Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au> Cc: Santosh Puranik <spuranik@nvidia.com> --- v2: - update copyright year to 2025 - improve dbg messages: use netdev, demote from _err, remove __func__ - pass gfp_t to mctp_usb_rx_queue, allocate first as GFP_KERNEL - release ->usbdev on disconnect - include MODULE_{DESCRIPTION,AUTHOR,DEVICE_TABLE} - more sensible driver->name - do rx stats update before netif_rx() - specify phys binding type via mctp_register_netdev() --- drivers/net/mctp/Kconfig | 10 ++ drivers/net/mctp/Makefile | 1 + drivers/net/mctp/mctp-usb.c | 368 ++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 379 insertions(+)