README.hardware

	It is hard running a backward compatible bare-metal OS on 
	Pi as things keep changing.

	Some of this is mitigated if you parse the device tree
	found on newer setups.

	Hardware Changes:
	=================
	+ On 1GB systems the IO_BASE moved 
		from 0x20000000 to 0x3f0000000
	+ Processor architecture changed from ARMv6 to ARMv7 to ARMv8
	+ On Pi3 the pl011 is hooked to bluetooth, so
		mini-uart is the default (pl011 is mapped to gpio 32/33)
	+ ACT LED:
		Model B was GPIO16, Active Low
		On 2/B+/A+ was GPIO47, Active High
		On 3B it is on an i2c/GPIO expander controlled by GPU
		On 3B+ it is on GPIO29, Active??
	+ When using mailbox to request firmware address, or the address
		with 0xc0000000 instead of 0x40000000

	+ Pi3 the Arasan MMC interface is enabled, and mapped to GPIOs 48-53,
		Used by wifi?
	+ Pi3 GPCLK2 is mapped to GPIO 43.
	+ Pi2/Pi3, firmware brings system up in HYP (hypervisor) mode
	+ Pi2/Pi3 have extra interrupt sources (PMU, etc)

	Processor Changes:
	==================
	+ On ARM1176 the "wfi" instruction is a nop, you need to
		use a MSR write to wait for instruction

	Firmware Changes:
	=================
	+ Older firmware, the framebuffer byte order was GRB instead of RGB
	+ Older firmware, the UART_CLOCK was 3MHz, it's 48MHz on newer
	+ Older firmware, did ATAGS, newer use device-tree
	+ Have issue on older-firmware loading kernel image >15MB
		not sure if bug or not





It should be possible to port to Model 2-B with minimal impact:
	+ Change mmio base address
	+ Possibly some IRQ handler changes?
	+ Cache and other low-level register support?
	+ Major changes needed to make it SMP capabale

Model 3-B support will be harder:
	+ ACT LED now on GPIO expander that is hooked to i2c-0
		and accessible through GPU mailbox
	+ UART now goes to bluetooth, need to handle mini-UART
		which lacks features and handles frequency
		changes poorly
	+ SD card I/O via different (undocumented?) interface
	+ Full 64-bit and SMP support will be