Raspberry Pi 4

The 9pi kernel source in /n/sources/contrib/miller/9/bcm on 9p.io has been
updated with (very limited) support for the new pi4.

Reminder: 'srv -nq tcp!9p.io sources /n/sources' to get access to this mirror.
Or point a browser at https://9p.io/sources/contrib/miller/9/bcm

I'm not suggesting anyone should rush out to buy a pi4 just yet.  Unlike
previous releases of the board, this incarnation has some very big changes,
so a lot of new driver writing will be needed before it's at all useful
under Plan 9.  It's quite daunting actually.

Quoting 9/bcm/words.pi4:

raspberry pi 4 -- work in progress!

broadcom 2838 SoC (based on 2711 cpu)
4 x cortex-a72 (1500Mhz, out-of-order pipeline)

Boots as far as the root prompt, but there are too many new
peripherals without driver support (or documentation so far)
to do anything useful.

- usb host sockets are all connected to new usb3 controller
- sdcard is connected to new emmc controller (can't use sdhost)
- ethernet is connected to new native gbe controller

Wifi is unchanged from 3B+, but currently fails intermittently
with emmc IO_RW_EXTENDED data corruption.

Needs firmware from 5 July 2019 or later.

config.txt for pi4 should include 'core_freq=250' for
the mini-uart console, and 'device_tree=' to ensure that
the loader passes an ATAG list to the kernel instead of
a device tree.