Programming the LAN9221 ethernet controller

	Programming the LAN9221 ethernet controller
	(on the Foenix A2560K in C)

	I am not a professional programmer and I had never tried to
	write anything like a 'device driver' or similar. I was faced
	with the challenge to write my 'own' driver for my Foenix
	A2560K modern retro machine. It doesn't come with an OS, you
	can't run Linux on it. So all you got is the bare machine. And
	I needed connectivity, preferably through Ethernet. So what do
	you do?

	For me, there are 2 important sources of information, of which
	one you'll find here. A assembly-wizard I met on the Foenix
	Retro Discord (by the screenname of 'gadget') created excellent
	Motorola 680x0 assembly code that can read and write to the
	controller. That code gave a pretty good insight into the way
	the LAN controller should be handled. But..it was assembly code
	and it was an extract from real purposed application code, so
	not really a reuseable library.

	The other resource is, of course, the original datasheet of the
	Microchip LAN9221 controller:

	The LAN9221 Datasheet

	As you can see from the datasheet the LAN9221 has an enormous
	array of registers that you must use to control it. The MAC and
	PHY modules can not be accessed directly, but need indirect
	read/write operations by commands that you have to give through
	separate command and data registers for these modules. We'll
	get to that later.

	The datasheet translates into an extensive header file, in
	which you'll recognize much of the datasheet's nomenclature:

	The lan9221.h header file

	The basics of controlling the 9221 are about reading and writing
	its registers. So the first thhin we need to be able to do,
	even before we are going to try to initialize the controller, we
	need to be able to read and write to its internal registers.

	Reading the registers: eth_reg_read()

	This should be very straightforward, right? What could be
	complicated about reading or writing to a known memory location?
	Well, there is one caveat: the endiness of the 32 bits (4 bytes)
	that you read or write.

	<explain the endiness here>

	Now that we know this, writing the registers should be pretty
	straightforward:

	Writing the registers: eth_reg_write()

	Now that we know how to read and write the registers, we are ready
	to initialize the module in our Foenix.

	To initialize the module, we do the following:
	step 1: wait until the module is 'up'
	step 2: check the byte-endiness by reading the BYTE_TEST register
	check: this register ALWAYS returns 0x87654321
	step 3: read the device ID (for fun)
	step 4: check the physical connection

	The following code-snippet implements this as eth_init():
	Initialize controller: eth_init()

	Set MAC address (eth_set_mac_address)