Re: [9front] PXE booting Rpi

[Daniel Morandini <danielmorandini@me.com>]

Hi people,
9pi2 is now getting its kernel configuration params! :D

Summary: add a “device_tree=“ line to your config.txt file if you
want your bcm pi to load cmdline.txt (9front 18e1e1c3aaf1, bootcode.bin
from [4] 8717598).

I spotted the “problem" in bcm/bootargs.c, `bootargsinit`.
```
void
bootargsinit(uintptr pa)
{
	uintptr len;

	/*
	 * kernel gets DTB/ATAGS pointer in R0 on entry
	 */
	if(pa != 0 && (len = cankaddr(pa)) != 0){
		void *va = KADDR(pa);
		if(parseatags(va, len) == 0 || parsedevtree(va, len) == 0) {
			return;
		}
	}

	/*
	 * /dev/reboot case, check CONFADDR
	 */
	if(parseatags(BOOTARGS, BOOTARGSLEN) != 0)
		plan9iniinit(BOOTARGS, 0);
}
```
Some notes. DTB/ATAGS pointer is passed to R2 on entry, not R0,
according to [1] and [2].
We call bootargsinit(0) though (main.c:87), so the first if statement
is never asserted, at least from the references within the bcm
folder.

Now according to the comment we should reach the second block only
when rebooting (?), but apparently this is not true. Quoting [3],

> The firmware will always try to load the DT and pass it to the
> kernel, since all kernels since rpi-4.4.y will not function without
> a DTB. You can override this by adding device_tree= in config.txt,
> which forces the use of ATAGs, which can be useful for simple
> "bare-metal" kernels.

And that was it! The second block in fact does not try to parse a
device tree but atags.

Why is bootargsinit called with pa=0? I think this function is not
working as intended anymore!

Cheers,
dan

[1] https://wiki.osdev.org/ARM_RaspberryPi
[2] http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html#appendix_tag_reference
[3] https://www.raspberrypi.org/documentation/configuration/device-tree.md#part3
[4] https://github.com/raspberrypi/firmware/blob/master/boot/bootcode.bin