[U-Boot] MIPS (mt7688): EBase change in U-Boot breaks Linux

[Stefan Roese]

Hi!

I've been hunting for a problem for quite some time, where Linux
hangs / crashes in userspace at some point on my MT7688 based
systems. I found that this problem can be avoided (worked around)
by not giving Linux the full memory (by using DT memory node fixup
or mem= kernel cmdline). When reducing this memory by the memory
used by U-Boot (stack pointer minus some KiB value as this is the
"lowest" memory used by U-Boot), then Linux runs just fine.

My first idea here was, that this issue is cache related (most
likely I-cache). But all tests and debugging in this area did not
fix this issue (even running with caches disabled).

Finally I found that this line in U-Boot makes Linux break:

arch/mips/lib/traps.c:

void trap_init(ulong reloc_addr)
	unsigned long ebase = gd->irq_sp;
	...
	write_c0_ebase(ebase);

This sets EBase to something like 0x87e9b000 on my system (128MiB).
And Linux then re-uses this value and copies the exceptions handlers
to this address, overwriting random code and leading to an unstable
system.

So my questions now is, how should this be handled on the MT7688
platform instead? One way would be to set EBase back to the
original value (0x80000000) before booting into Linux. Another
solution would be to add some Linux code like board_ebase_setup()
to the MT7688 Linux port.

Since I'm still no real MIPS expert yet, I would really like to get
some advise here on how to best solve this issue. Maybe I missed
something. Comments?

Thanks,
Stefan