[PATCH] PPPoL2TP: Add more code snippets

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

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 described, so that people were thinking
it was not supported, while it actually is.

Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>

---
 Documentation/networking/l2tp.rst |   59 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 56 insertions(+), 3 deletions(-)

--- a/Documentation/networking/l2tp.rst
+++ b/Documentation/networking/l2tp.rst
@@ -387,11 +387,12 @@ Sample userspace code:
   - Create session PPPoX data socket::
 
         struct sockaddr_pppol2tp sax;
-        int fd;
+        int ret;
 
         /* Note, the tunnel socket must be bound already, else it
          * will not be ready
          */
+        int session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
         sax.sa_family = AF_PPPOX;
         sax.sa_protocol = PX_PROTO_OL2TP;
         sax.pppol2tp.fd = tunnel_fd;
@@ -406,12 +407,64 @@ 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 ) {
                 return -errno;
         }
         return 0;
 
+  - Create PPP channel::
+
+        int chindx;
+        ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+        if (ret < 0)
+                return -errno;
+
+        int ppp_chan_fd = open("/dev/ppp", O_RDWR);
+
+        ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx);
+        if (ret < 0)
+                return -errno;
+
+Non-data PPP frames will be available for read on `ppp_chan_fd`.
+
+  - Create PPP interface::
+
+        int ppp_if_fd = open("/dev/ppp", O_RDWR);
+
+        int ifunit;
+        ret = ioctl(ppp_if_fd, PPPIOCNEWUNIT, &ifunit);
+        if (ret < 0)
+                return -errno;
+
+        ret = ioctl(ppp_chan_fd, PPPIOCCONNECT, ifunit);
+        if (ret < 0)
+                return -errno;
+
+The ppp<ifunit> interface can then be configured as usual with SIOCSIFMTU,
+SIOCSIFADDR, SIOCSIFDSTADDR, SIOCSIFNETMASK, and activated by setting IFF_UP
+with SIOCSIFFLAGS
+
+  - Tunnel switching is supported by bridging channels::
+
+        int chindx;
+        ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+        if (ret < 0)
+                return -errno;
+
+        int chindx2;
+        ret = ioctl(session_fd2, PPPIOCGCHAN, &chind2x);
+        if (ret < 0)
+                return -errno;
+
+        int ppp_chan_fd = open("/dev/ppp", O_RDWR);
+
+        ret = ioctl(ppp_chan_fd, PPPIOCBRIDGECHAN, &chindx2);
+        if (ret < 0)
+                return -errno;
+
+        close(ppp_chan_fd);
+
 Old L2TPv2-only API
 -------------------