This file provides a simple description of how to write a low-level,
hardware dependent ethernet driver.

The basic idea is that there is a high-level driver (which is only
code/functions) that is part of the stack.  There will be one or more
low-level driver tied to the actual network hardware.  Each of these
drivers contains one or more driver instances.  The principal idea is
that the low-level drivers know nothing of the details of the stack that
will be using them.  Thus, the same driver can be used by the eCos
supported TCP/IP stack, or any other, with no changes.

A driver instance is contained within a "struct eth_drv_sc".

   struct eth_drv_funs {
      // Initialize hardware (including startup)
      void (*start)(struct eth_drv_sc *sc,
                    unsigned char *enaddr);
      // Shut down hardware
      void (*stop)(struct eth_drv_sc *sc);
      // Control interface
      int (*control)(struct eth_drv_sc *sc,
                     unsigned long cmd,
                     void *data,
                     int len);
      // Query interface - can a packet be sent?
      int (*can_send)(struct eth_drv_sc *sc);
      // Send a packet of data
      void (*send)(struct eth_drv_sc *sc,
                   struct eth_drv_sg *sg_list,
                   int sg_len,
                   int total_len,
                   unsigned long key);
      // Receive [unload] a packet of data
      void (*recv)(struct eth_drv_sc *sc,
                   struct eth_drv_sg *sg_list,
                   int sg_len);
  };

  struct eth_drv_sc {
      struct eth_drv_funs *funs;
      void                *driver_private;
      const char          *dev_name;
      struct arpcom       sc_arpcom; /* ethernet common */
  };

You create one of these instances using the "ETH_DRV_SC()" macro which
sets up the structure, including the prototypes for the functions, etc.
By doing things this way, if the internal design of the ethernet drivers
changes (e.g. we need to add a new low-level implementation function),
existing drivers will no longer compile until updated.  This is much
better than to have all of the definitions in the low-level drivers
themselves and have them be [quietly] broken if the interfaces change.

The "magic" which gets the drivers started [and indeed, linked] is
similar to what is used for the I/O subsystem. [Note: I may try and
make it part of the I/O subsystem later.]  This is done using the
"NETDEVTAB_ENTRY()" macro, which defines an initialization function
and the basic data structures for the low-level driver.

  typedef struct cyg_netdevtab_entry {
      const char        *name;
      bool             (*init)(struct cyg_netdevtab_entry *tab);
      void              *device_instance;
      unsigned long     status;
  } cyg_netdevtab_entry_t;

The "device_instance" entry here would point to the "struct
eth_drv_sc" entry previously defined.  This allows the network driver
setup to work with any class of driver, not just ethernet drivers.  In
the future, there will surely be serial PPP drivers, etc.  These will
use the "NETDEVTAB" setup to create the basic driver, but they will
most likely be built on top of other high-level device driver layers.

So, the bottom line is that a hardware driver will have a template
(boilerplate) which looks like this:

	#include <cyg/infra/cyg_type.h>
	#include <cyg/hal/hal_arch.h>
	#include <cyg/infra/diag.h>
	#include <cyg/hal/drv_api.h>
	#include <netdev.h>
	#include <eth_drv.h>

	ETH_DRV_SC(DRV_sc,
	           0,             // No driver specific data needed
	           "eth0",        // Name for this interface
	           HRDWR_start,
	           HRDWR_stop,
	           HRDWR_control,
	           HRDWR_can_send
	           HRDWR_send,
	           HRDWR_recv);

	NETDEVTAB_ENTRY(DRV_netdev, 
	                "DRV", 
	                DRV_HRDWR_init, 
	                &DRV_sc);

This, along with the referenced functions, completely define the driver.
Extensibility note: if one needed the same low-level driver to handle
multiple similar hardware interfaces, you would need multiple invocations
of the "ETH_DRV_SC()/NETDEVTAB_ENTRY()" macros.  You would add a pointer
to some instance specific data, e.g. containing base addresses, interrupt
numbers, etc, where the "0, // No driver specific data" is currently.

Now a quick waltz through the functions.  This discussion will use the
generic names from above.

static bool DRV_HDWR_init(struct cyg_netdevtab_entry *tab)
==========================================================

This function is called as part of system initialization.  Its primary
function is to decide if the hardware [as indicated via
tab->device_instance] is working and if the interface needs to be made
available in the system.  If this is the case, this function needs to
finish with a call to the ethernet driver function:

	eth_drv_init((struct eth_drv_sc *)tab->device_instance,
	             unsigned char *enaddr);

where 'enaddr' is a pointer to the ethernet station address for this
unit.  Note: the ethernet station address is supposed to be a
world-unique, 48 bit address for this particular ethernet interface.
Typically it is provided by the board/hardware manufacturer in ROM.

static void
HRDWR_start(struct eth_drv_sc *sc, unsigned char *enaddr, int flags)
====================================================================

This function is called, perhaps much later than system initialization
time, when the system (an application) is ready for the interface to
become active.  The purpose of this function is to set up the hardware
interface to start accepting packets from the network and be able to
send packets out.  Note: this function will be called whenever the
up/down state of the logical interface changes, e.g. when the IP address
changes.  This may occur more than one time, so this function needs to
be prepared for that case.

FUTURE: the "flags" field (currently unused) may be used to tell the
function how to start up, e.g. whether interrupts will be used,
selection of "promiscuous" mode etc.

static void HRDWR_stop(struct eth_drv_sc *sc)
=============================================

This function is the inverse of "start".  It should shut down the
hardware and keep it from interacting with the physical network.

static int
HRDWR_control(struct eth_drv_sc *sc, unsigned long key, void *data, int len)
============================================================================

This function is used to perform low-level "control" operations on the
interface.  These operations would be initiated via 'ioctl()' in the BSD
stack, and would be anything that would require the hardware setup to
possibly change (i.e.  cannot be performed totally by the
platform-independent layers).

Current operations:

ETH_DRV_SET_MAC_ADDRESS:
  This function sets the ethernet station address (ESA or MAC) for the
  device.  Normally this address is kept in non-volatile memory and is
  unique in the world.  This function must at least set the interface to
  use the new address.  It may also update the NVM as appropriate.

This function should return zero if the specified operation was
completed successfully.  It should return non-zero if the operation
could not be performed, for any reason.

static int HRDWR_can_send(struct eth_drv_sc *sc)
================================================

This function is called to determine if it is possible to start the
transmission of a packet on the interface.  Some interfaces will allow
multiple packets to be "queued" and this function allows for the highest
possible utilization of that mode.

Return the number of packets which could be accepted at this time, zero
implies that the interface is saturated/busy.

static void
HRDWR_send(struct eth_drv_sc *sc, struct eth_drv_sg *sg_list, int sg_len,
                       int total_len, unsigned long key)
=========================================================================

This function is used to send a packet of data to the network.  It is
the responsibility of this function to somehow hand the data over to the
hardware interface.  This will most likely require copying, but just the
address/length values could be used by smart hardware.

NOTE: All data in/out of the driver is specified via a "scatter-gather"
list.  This is just an array of address/length pairs which describe
sections of data to move (in the order given by the array).

Once the data has been successfully sent by the interface (or if an
error occurs), the driver should call 'eth_drv_tx_done()' using the
specified 'key'.  Only then will the upper layers release the resources
for that packet and start another transmission.

FUTURE: This function may be extended so that the data need not be
copied by having the function return a "disposition" code (done, send
pending, etc).  At this point, you should move the data to some "safe"
location before returning.

static void
HRDWR_recv(struct eth_drv_sc *sc, struct eth_drv_sg *sg_list, int sg_len)
=========================================================================

This function is actually a call back, only invoked after the
upper-level function
  eth_drv_recv(struct eth_drv_sc *sc, int total_len)
has been called.  This upper level function is called by the hardware
driver when it knows that a packet of data is available on the
interface.  The 'eth_drv_recv()' function then arranges network buffers
and structures for the data and then calls "HRDWR_recv()" to actually
move the data from the interface.


Upper layer functions - called by drivers
=========================================

These functions are defined by the upper layers (machine independent) of
the networking driver support.  They are present to hide the interfaces
to the actual networking stack so that the hardware drivers may possibly
be used by any network stack implementation.

These functions require a pointer to a "struct eth_drv_sc" table which
describes the interface at a logical level.  It is assumed that the
driver [lowest level hardware support] will keep track of this pointer
so it may be passed "up" as appropriate.

  struct eth_drv_sc {
      struct eth_drv_funs *funs; // Pointer to hardware functions (see above)
      void                *driver_private;   // Device specific data
      const char          *dev_name;
      struct arpcom       sc_arpcom; // ethernet common
  };

This structure is created, one per logical interface, via ETH_DRV_SC macro.

void eth_drv_init(struct eth_drv_sc *sc, unsigned char *enaddr)
===============================================================

This function establishes the device at initialization time.  The
hardware should be totally intialized (not "started") when this function
is called.

void eth_drv_tx_done(struct eth_drv_sc *sc, unsigned long key, int status)
==========================================================================

This function is called when a packet completes transmission on the
interface.  The 'key' value must be one of the keys provided to
"HRDWR_send()" above.  The value 'status' should be non-zero (currently
undefined) to indicate that an error occurred during the transmission.

void eth_drv_recv(struct eth_drv_sc *sc, int len)
=================================================

This function is called to indicate that a packet of length 'len' has
arrived at the interface.  The callback "HRDWR_recv()" function
described above will be used to actually unload the data from the
interface into buffers used by the machine independent layers.



Calling graph for Transmit and Receive
--------------------------------------

It may be worth clarifying further the flow of control in the transmit
and receive cases, where the hardware driver does use interrupts and so
DSRs to tell the "foreground" when something asynchronous has occurred.

Transmit:
  Foreground task calls into network stack to send a packet (or the
  stack decides to send a packet in response to incoming traffic).
  The driver calls the HRDWR_can_send() function in the hardware driver.
    HRDWR_can_send() returns the number of available "slots" in which it
    can store a pending transmit packet.
  If it cannot send at this time, the packet is queued outside the
  hardware driver for later; in this case, the hardware is already busy
  transmitting, so expect an interrupt as described below for completion
  of the packet currently outgoing.
  If it can send right now, HRDWR_send() is called.
    HRDWR_send() copies the data into special hardware buffers, or
    instructs the hardware to "send that".
    It also remembers the key that is associated with this tx request.
  these calls return.
  ...

  Asynchronously, the hardware makes an interrupt to say "transmit is
  done"; the ISR quietens the interrupt source in the hardware and
  requests that the associated DSR be run.
  The DSR realizes that a transmit request has completed, and calls
  eth_drv_tx_done() with the same key that it remembered for this tx.
    eth_drv_tx_done() uses the key to find the resources associated with
    this transmit request; thus the stack knows that the transmit has
    completed and its resources can be freed.
    eth_drv_tx_done() also enquires whether HRDWR_can_send() now says
    "yes, we can send" and if so, dequeues a further transmit request
    which may have been queued as described above.  If so:
      HRDWR_send() copies the data into the hardware buffers, or
      instructs the hardware to "send that" and remembers the new key.
  these calls return to the DSR and thus to the foreground.
  ...


Receive:
  ...

  Asynchronously, the hardware makes an interrupt to say "there is ready
  data in a receive buffer"; the ISR quietens the interrupt source in
  the hardware and requests that the associated DSR be run.
  The DSR realizes that there is data ready and calls eth_drv_recv()
  with the length of the data that is available.
    eth_drv_recv() prepares a set of scatter-gather buffers that can
    accommodate that data.
    It then calls back into the hardware driver routine HRDWR_recv().
       HRDWR_recv() must copy the data from the hardware's buffers into
       the scatter-gather buffers provided, and return.
    eth_drv_recv() sends the new packet up the network stack and returns.
  Back in the DSR now, the driver cleans the receive buffer and returns
  it to the hardware's control, available to receive another packet from
  the network.
  The DSR returns to the foreground.
  ...


