[PATCHv2] 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

 Documentation/networking/l2tp.rst |   97 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 93 insertions(+), 4 deletions(-)

--- a/Documentation/networking/l2tp.rst
+++ b/Documentation/networking/l2tp.rst
@@ -387,11 +387,16 @@ Sample userspace code:
   - Create session PPPoX data socket::
 
         struct sockaddr_pppol2tp sax;
-        int fd;
+        int session_fd;
+        int ret;
 
         /* Note, the tunnel socket must be bound already, else it
          * will not be ready
          */
+        session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
+        if (session_fd < 0)
+                return -errno;
+
         sax.sa_family = AF_PPPOX;
         sax.sa_protocol = PX_PROTO_OL2TP;
         sax.pppol2tp.fd = tunnel_fd;
@@ -406,11 +411,95 @@ 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::
+
+        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 0;
+
+        return ppp_chan_fd;
+
+Non-data PPP frames will be available for read on `ppp_chan_fd`.
+
+  - Create PPP interface::
+
+        int ppp_if_fd;
+        int ifunit = -1;
+
+        ppp_if_fd = open("/dev/ppp", O_RDWR);
+        if (ppp_chan_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 ppp_chan_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.
+
+  - L2TP session bridging (also called L2TP tunnel switching or L2TP multihop)
+is supported by bridging the ppp channels of the two L2TP sessions to be
+bridged::
+
+        int chindx1;
+        int chindx2;
+        int ppp_chan_fd;
+
+        ret = ioctl(session_fd1, PPPIOCGCHAN, &chindx1);
+        if (ret < 0)
+                return -errno;
+
+        ret = ioctl(session_fd2, PPPIOCGCHAN, &chind2x);
+        if (ret < 0)
+                return -errno;
+
+        ppp_chan_fd = open("/dev/ppp", O_RDWR);
+        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;
+
+See more details for the PPP side in ppp_generic.rst.
 
 Old L2TPv2-only API
 -------------------