How to get a memory mapped serial console

Memory mapped uart

So, a friend was bringing up FreeBSD on a new system. It didn't have a traditionally mapped UART, but a memory mapped one, like we have in the embedded world. After stumbling around, I thought I'd document how to get a serial port in a situation like this.

First, he had linux running. It recognized the serial port. And it worked in UEFI.

In Linux, we first found the ttys:

% dmesg | grep tty
AMDI0020:00:ttyS4 at MMIO 0xfedc9000 (irq = 3, base_baud = 30000000) is a 16550A
AMDI0020:01:ttyS5 at MMIO 0xfedca000 (irq = 4, base_baud = 30000000) is a 16550A
ttyS4 at MMIO 0xfedc9000 (irq = 3, base_baud = 30000000) is a 16550A
ttyS5 at MMIO 0xfedca000 (irq = 4, base_baud = 30000000) is a 16550A

So, this is weird. Or rather, it's normal if you are used to ARM s16550's memory mapped, or some PCIe, PCI and CardBus serial cards.

It turns out that even if we don't have a driver for this that attaches automatically (uart works, but needs some glue on this platform), you can have a serial port console none-the-less. At the boot loader OK prompt:

OK set hw.uart.console="mm:0xfedc9000,rs:2"

What's the "rs:2"? If you look at the source it's Register Shift. It means that the cadence of registers isn't 1, but 4. It also means that each register is 32-bits, but only the lower 8 bits are valid. That's also typical in ARM and MIPS embedded processors.

So now you know.