[PATCHv4] PPPoL2TP: Add more code snippets

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

The existing documentation was not telling that one has to create a PPP
channel and a PPP interface to get PPPoL2TP data offloading working.

Also, tunnel switching was not mentioned, so that people were thinking
it was not supported, while it actually is.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

---
Difference from v1:
- follow kernel coding style
- check for failures
- also mention netlink and ip for configuring the link
- fix bridging channels

Difference from v2:
- fix text alignment

Difference from v3:
- fix some variables references
- explicit inputs of the code snippets
- explicit that bridging is supported for l2tp with PPP pseudowire type.
- explicit that after bridging only the pppox sockets need to be kept
- explicit that bridging can also be done with other types of ppp
  channels

--- a/Documentation/networking/l2tp.rst
+++ b/Documentation/networking/l2tp.rst
@@ -386,12 +386,17 @@ Sample userspace code:
 
   - Create session PPPoX data socket::
 
+        /* Input: the L2TP tunnel UDP socket `tunnel_fd`, which needs to be
+         * bound already, otherwise it will not be ready. */
+
         struct sockaddr_pppol2tp sax;
-        int fd;
+        int session_fd;
+        int ret;
+
+        session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
+        if (session_fd < 0)
+                return -errno;
 
-        /* Note, the tunnel socket must be bound already, else it
-         * will not be ready
-         */
         sax.sa_family = AF_PPPOX;
         sax.sa_protocol = PX_PROTO_OL2TP;
         sax.pppol2tp.fd = tunnel_fd;
@@ -406,11 +411,117 @@ Sample userspace code:
         /* session_fd is the fd of the session's PPPoL2TP socket.
          * tunnel_fd is the fd of the tunnel UDP / L2TPIP socket.
          */
-        fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
-        if (fd < 0 ) {
+        ret = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
+        if (ret < 0 ) {
+                close(session_fd);
+                return -errno;
+        }
+
+        return session_fd;
+
+L2TP control packets will still be available for read on `tunnel_fd`.
+
+  - Create PPP channel::
+
+        /* Input: the session PPPoX data socket session_fd which was created as
+         * described above. */
+
+        int chindx;
+        int ppp_chan_fd;
+
+        ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+        if (ret < 0)
+                return -errno;
+
+        ppp_chan_fd = open("/dev/ppp", O_RDWR);
+        if (ppp_chan_fd < 0)
+                return -errno;
+
+        ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx);
+        if (ret < 0) {
+                close(ppp_chan_fd);
+                return -errno;
+        }
+
+        return ppp_chan_fd;
+
+LCP PPP frames will be available for read on `ppp_chan_fd`.
+
+  - Create PPP interface::
+
+        /* Input: the PPP channel ppp_chan_fd which was created as described
+         * above */
+
+        int ppp_if_fd;
+        int ifunit = -1;
+
+        ppp_if_fd = open("/dev/ppp", O_RDWR);
+        if (ppp_if_fd < 0)
+                return -errno;
+
+        ret = ioctl(ppp_if_fd, PPPIOCNEWUNIT, &ifunit);
+        if (ret < 0) {
+                close(ppp_if_fd);
+                return -errno;
+        }
+
+        ret = ioctl(ppp_chan_fd, PPPIOCCONNECT, ifunit);
+        if (ret < 0) {
+                close(ppp_if_fd);
                 return -errno;
         }
-        return 0;
+
+        return ppp_if_fd;
+
+IPCP/IPv6CP PPP frames will be available for read on `ppp_if_fd`.
+
+The ppp<ifunit> interface can then be configured as usual with netlink's
+RTM_NEWLINK, RTM_NEWADDR, RTM_NEWROUTE, or ioctl's SIOCSIFMTU, SIOCSIFADDR,
+SIOCSIFDSTADDR, SIOCSIFNETMASK, SIOCSIFFLAGS, or with the `ip` command.
+
+  - Bridging L2TP sessions which have PPP pseudowire types (this is also called
+    L2TP tunnel switching or L2TP multihop) is supported by bridging the ppp
+    channels of the two L2TP sessions to be bridged::
+
+        /* Input: the session PPPoX data sockets session_fd1 and session_fd2
+         * which were created as described further above. */
+
+        int chindx1;
+        int chindx2;
+        int ppp_chan_fd;
+
+        ret = ioctl(session_fd1, PPPIOCGCHAN, &chindx1);
+        if (ret < 0)
+                return -errno;
+
+        ret = ioctl(session_fd2, PPPIOCGCHAN, &chindx2);
+        if (ret < 0)
+                return -errno;
+
+        ppp_chan_fd = open("/dev/ppp", O_RDWR);
+        if (ppp_chan_fd < 0) {
+                return -errno;
+        }
+
+        ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx1);
+        if (ret < 0) {
+                close(ppp_chan_fd);
+                return -errno;
+        }
+
+        ret = ioctl(ppp_chan_fd, PPPIOCBRIDGECHAN, &chindx2);
+        close(ppp_chan_fd);
+        if (ret < 0)
+                return -errno;
+
+It can be noted that in this case no PPP interface is needed, and the PPP
+channel does not need to be kept open.  Only the session PPPoX data sockets need
+to be kept open.
+
+More generally, it is also possible in the same way to e.g. bridge a PPPol2tp
+ppp channel with other types of ppp channels, such as PPPoE.
+
+See more details for the PPP side in ppp_generic.rst.
 
 Old L2TPv2-only API
 -------------------