net: add devmem TCP TX documentation

Add documentation outlining the usage and details of the devmem TCP TX
API.

Signed-off-by: Mina Almasry <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

Documentation/networking/devmem.rst

···
     https://lore.kernel.org/netdev/[e-mail]/
 
 
-Interface
-=========
+RX Interface
+============
 
 
 Example
 -------
 
-tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up
-the RX path of this API.
+./tools/testing/selftests/drivers/net/hw/ncdevmem:do_server shows an example of
+setting up the RX path of this API.
 
 
 NIC Setup
···
 
 (a) an internal kernel leak bug.
 (b) the user passed more than 1024 frags.
+
+TX Interface
+============
+
+
+Example
+-------
+
+./tools/testing/selftests/drivers/net/hw/ncdevmem:do_client shows an example of
+setting up the TX path of this API.
+
+
+NIC Setup
+---------
+
+The user must bind a TX dmabuf to a given NIC using the netlink API::
+
+        struct netdev_bind_tx_req *req = NULL;
+        struct netdev_bind_tx_rsp *rsp = NULL;
+        struct ynl_error yerr;
+
+        *ys = ynl_sock_create(&ynl_netdev_family, &yerr);
+
+        req = netdev_bind_tx_req_alloc();
+        netdev_bind_tx_req_set_ifindex(req, ifindex);
+        netdev_bind_tx_req_set_fd(req, dmabuf_fd);
+
+        rsp = netdev_bind_tx(*ys, req);
+
+        tx_dmabuf_id = rsp->id;
+
+
+The netlink API returns a dmabuf_id: a unique ID that refers to this dmabuf
+that has been bound.
+
+The user can unbind the dmabuf from the netdevice by closing the netlink socket
+that established the binding. We do this so that the binding is automatically
+unbound even if the userspace process crashes.
+
+Note that any reasonably well-behaved dmabuf from any exporter should work with
+devmem TCP, even if the dmabuf is not actually backed by devmem. An example of
+this is udmabuf, which wraps user memory (non-devmem) in a dmabuf.
+
+Socket Setup
+------------
+
+The user application must use MSG_ZEROCOPY flag when sending devmem TCP. Devmem
+cannot be copied by the kernel, so the semantics of the devmem TX are similar
+to the semantics of MSG_ZEROCOPY::
+
+	setsockopt(socket_fd, SOL_SOCKET, SO_ZEROCOPY, &opt, sizeof(opt));
+
+It is also recommended that the user binds the TX socket to the same interface
+the dma-buf has been bound to via SO_BINDTODEVICE::
+
+	setsockopt(socket_fd, SOL_SOCKET, SO_BINDTODEVICE, ifname, strlen(ifname) + 1);
+
+
+Sending data
+------------
+
+Devmem data is sent using the SCM_DEVMEM_DMABUF cmsg.
+
+The user should create a msghdr where,
+
+* iov_base is set to the offset into the dmabuf to start sending from
+* iov_len is set to the number of bytes to be sent from the dmabuf
+
+The user passes the dma-buf id to send from via the dmabuf_tx_cmsg.dmabuf_id.
+
+The example below sends 1024 bytes from offset 100 into the dmabuf, and 2048
+from offset 2000 into the dmabuf. The dmabuf to send from is tx_dmabuf_id::
+
+       char ctrl_data[CMSG_SPACE(sizeof(struct dmabuf_tx_cmsg))];
+       struct dmabuf_tx_cmsg ddmabuf;
+       struct msghdr msg = {};
+       struct cmsghdr *cmsg;
+       struct iovec iov[2];
+
+       iov[0].iov_base = (void*)100;
+       iov[0].iov_len = 1024;
+       iov[1].iov_base = (void*)2000;
+       iov[1].iov_len = 2048;
+
+       msg.msg_iov = iov;
+       msg.msg_iovlen = 2;
+
+       msg.msg_control = ctrl_data;
+       msg.msg_controllen = sizeof(ctrl_data);
+
+       cmsg = CMSG_FIRSTHDR(&msg);
+       cmsg->cmsg_level = SOL_SOCKET;
+       cmsg->cmsg_type = SCM_DEVMEM_DMABUF;
+       cmsg->cmsg_len = CMSG_LEN(sizeof(struct dmabuf_tx_cmsg));
+
+       ddmabuf.dmabuf_id = tx_dmabuf_id;
+
+       *((struct dmabuf_tx_cmsg *)CMSG_DATA(cmsg)) = ddmabuf;
+
+       sendmsg(socket_fd, &msg, MSG_ZEROCOPY);
+
+
+Reusing TX dmabufs
+------------------
+
+Similar to MSG_ZEROCOPY with regular memory, the user should not modify the
+contents of the dma-buf while a send operation is in progress. This is because
+the kernel does not keep a copy of the dmabuf contents. Instead, the kernel
+will pin and send data from the buffer available to the userspace.
+
+Just as in MSG_ZEROCOPY, the kernel notifies the userspace of send completions
+using MSG_ERRQUEUE::
+
+        int64_t tstop = gettimeofday_ms() + waittime_ms;
+        char control[CMSG_SPACE(100)] = {};
+        struct sock_extended_err *serr;
+        struct msghdr msg = {};
+        struct cmsghdr *cm;
+        int retries = 10;
+        __u32 hi, lo;
+
+        msg.msg_control = control;
+        msg.msg_controllen = sizeof(control);
+
+        while (gettimeofday_ms() < tstop) {
+                if (!do_poll(fd)) continue;
+
+                ret = recvmsg(fd, &msg, MSG_ERRQUEUE);
+
+                for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) {
+                        serr = (void *)CMSG_DATA(cm);
+
+                        hi = serr->ee_data;
+                        lo = serr->ee_info;
+
+                        fprintf(stdout, "tx complete [%d,%d]\n", lo, hi);
+                }
+        }
+
+After the associated sendmsg has been completed, the dmabuf can be reused by
+the userspace.
+
 
 Implementation & Caveats
 ========================