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

[Horatiu Vultur]

Hi Stefan,

The 12/12/2018 12:41, Stefan Roese wrote:
> 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.

I am glad that it worked.

> 
> BTW: Should CPU_MIPSR2_IRQ_EI probably also be set?

Well, it depends on the interrupt controller that you have. In our case
we didn't set it. But I am not an MIPS expert, so I may be wrong.

> > 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
> > > _______________________________________________
> > > U-Boot mailing list
> > > U-Boot at lists.denx.de
> > > https://lists.denx.de/listinfo/u-boot
> > 
> 
> Viele Grüße,
> Stefan
> 
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-- 
/Horatiu