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

[Horatiu Vultur]

Hi Stefan,

Is your Linux Kernel compile with CONFIG_CPU_MIPSR2_IRQ_VI? Because we
had similar issue with two of our boards(Ocelot and Luton).

In our case the problem was that that Linux Kernel didn't reserve memory
for the addresses pointed by ebase register and then later the kernel
used this address overwriting the interupt vector, that lead to random
crash.

The 12/12/2018 09:18, Stefan Roese wrote:
> 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
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-- 
/Horatiu