PORTING

# $Id$

Adding netmap support to network device drivers
------------------------------------------------

Netmap requires some small modifications to device drivers
to support the new API. You will need to add small patches
in 3-4 places in the original source, and implement typically
5 new functions.

Device driver patches
------------------------
+ in the initial part of the source, after the device-specific
  headers and prototypes have been declared, add the following
    <pre>
	+#if defined(DEV_NETMAP) || defined(CONFIG_NETMAP) || defined(CONFIG_NETMAP_MODULE)
	+#include <dev/netmap/if_re_netmap.h>
	+#endif /* !DEV_NETMAP */
    </pre>
    The place is typically ... in FreeBSD, and
    ... on Linux.

    The header really contains the new functions that implement
    the netmap API. Including them inline simplifies the building
    as it does not require to insert additional dependencies in the
    build system.

    On FreeBSD DEV_NETMAP is sufficient to detect whether netmap extensions
    should be compiled in, whereas CONFIG_NETMAP and CONFIG_NETMAP_MODULE
    are the Linux equivalent.

    If a driver is made of multiple source files, you will need to include
    the additional header in all the (few) patched files, preferably using
    a macro such as NETMAP_FOO_MAIN to indicate the file where the
    new functions should be compiled in.

+ near the end of the attach routine, once the ifnet/net_device structure
  has been filled and initialized, add
    <pre>
	+#ifdef DEV_NETMAP
	+	foo_netmap_attach(adapter);
	+#endif /* DEV_NETMAP */
    </pre>
  The argument is either the ifnet or the private device descriptor.  
  This is in foo_attach() on FreeBSD, and somewhere in the path of
  XXX foo_open() in Linux

+ near the code called on device removal, add
    <pre>
	+#ifdef DEV_NETMAP
	+	netmap_detach(ifp);
	+#endif /* DEV_NETMAP */
    </pre>

+ after the tx/rx rings have been initialized, add a patch like this:
    <pre>
	+#ifdef DEV_NETMAP
	+	foo_netmap_config(priv);
	+#endif /* DEV_NETMAP */
    </pre>
    The argument is typically the private device descriptor, or even
    the struct ifnet/net_device.

+ in the interrupt dispatch routines, something like
    <pre>
	+#ifdef DEV_NETMAP
	+       int dummy;
	+       if (netmap_rx_irq(adapter->netdev, rx_ring->queue_index, &dummy))
	+               return true;
	+#endif /* DEV_NETMAP */
	...
	+#ifdef DEV_NETMAP
	+       if (netmap_tx_irq(adapter->netdev, tx_ring->queue_index))
	+               return true; /* seems to be ignored */
	+#endif /* DEV_NETMAP */
     </pre>
     to skip the normal processing and instead wake up the process in
     charge of doing I/O

New functions
----------------
The new functions serve to register the netmap-enabled device driver,
support the enable/disable of netmap mode, attach netmap buffers to the
NIC rings, and finally implement the handlers (*_txsync(), *_rxsync())
called by the system calls.

* foo_netmap_attach()
    This is a relatively mechanical function. The purpose is to fetch from
    the device descriptor information on the number of rings and buffers,
    the way locks are used, and invoke netmap_attach().

* foo_netmap_config()
    This function is in charge of (over)writing the NIC rings with
    pointers to the netmap buffers. Although this is device dependent,
    we can often ignore the locking issue and expect that the locking is
    already taken care of by the caller.

    foo_netmap_config() only needs to run if the card is in netmap mode.
    A quick way to check is to call netmap_ring_init() on one of the rings,
    if the function returns NULL we can immediately exit.
    Otherwise, we should run a couple of nested loops (on the rings,
    and then on the buffers) to fill the NIC descriptors with the
    addresses of the (preallocated) netmap buffers.

    For the TX rings this can even be a no-op because these rings are
    typically uninitialized, and the pointers can be overridden in the
    txsync() routine.

    For the receive ring, the operation is more critical because the
    buffers should be available by the time the NIC is enabled.

    Note that the device driver typically maintains head and tail pointers
    to indicate which buffers are used. It might be convenient to retain
    these indexes because may of the support routines, watchdogs etc.
    depends on their values.

    We should note that, especially on the receive ring, there might be
    an offset between the indexes used in the netmap ring and those used
    in the NIC ring (which might even be non-contiguous).

* foo_netmap_reg()
    support entering/exiting of netmap mode. Typically, lock, stop the device,
    set/clear the netmap flag, and restart the device.
    An unfortunate side effect of stopping and restarting the device is that
    in many drivers the link is reinitialized, causing long delays for the
    speed negotiations and spanning tree setup.


* foo_netmap_txsync()

* foo_netmap_rxsync()