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

[Stefan Roese]

Hi Horatiu Vultur,

On 12.12.18 12:21, Horatiu Vultur wrote:
> Is your Linux Kernel compile with CONFIG_CPU_MIPSR2_IRQ_VI? Because we
> had similar issue with two of our boards(Ocelot and Luton).

No, its not configured for this MT7688 / RAMIPS SoC. Enabling this
option does fix this issue. Many thanks for the suggestion.

BTW: Should CPU_MIPSR2_IRQ_EI probably also be set?
  
> 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.

Exactly what we've observed here. It took quite some debugging to
finally find the root-cause for this.

Thanks,
Stefan
  
> 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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>> U-Boot at lists.denx.de
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> 

Viele Grüße,
Stefan

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