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

[Stefan Roese]

Hi Paul,

On 13.12.18 20:47, Paul Burton wrote:
> On Thu, Dec 13, 2018 at 03:23:39PM +0100, Daniel Schwierzeck wrote:
>>>>>>> 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.
>> %
>>>> I could also prepare a U-Boot patch to restore the original ebase value before
>>>> handing the control over to the OS.
>>>
>>> I'm not so sure, if overwriting 0x80000000 (default value of EBase on
>>> this SoC) with the exception handler is allowed. Is this address "zero"
>>> handled somewhat specific in MIPS Linux? AFAICT, the complete DDR
>>> area on my platform (0x8000.0000 - 0x87ff.ffff) is available for Linux.
>>> So allocating some memory for this exception handler seems the right
>>> way to go to me.
>>
>> maybe that's why some platforms define a load address of 0x80002000 or similar
>> to protect this area somehow.
> 
> Does this Linux patch help by any chance?
> 
> https://git.linux-mips.org/cgit/linux-mti.git/commit/?h=eng-v4.20&id=39e4d339a4540b66e9d9a8ea0da9ee41a21473b4
> 
> I'm not sure I remember why I didn't get that upstreamed yet, I probably
> wanted to research what other systems were doing... Speaking for Malta,
> the kernel's board support has reserved the start of kseg0 for longer
> than I've been involved.

No, this patch does not solve this issue (bootup still hangs or crashes
while mounting the rootfs). I can only assume that its too late to try
to reserve this memory region as the memblock_reserve() call returns 0
(no error).
  
> An alternative would be for Linux to allocate a page for use with the
> exception vectors using memblock, and ignore the EBase value U-Boot left
> us with. But just marking the area U-Boot used as reserved ought to do
> the trick, and has the advantage of ensuring U-Boot's vectors don't get
> overwritten before Linux sets up its own which sometimes allows U-Boot
> to provide some useful output.

I agree that re-using the U-Boot value would be optimal for boot-time
error printing. But this does not seem to work on our platform AFAICT.
So how to proceed? Should I enable CONFIG_CPU_MIPSR2_IRQ_VI or #define
"cpu_has_veic" to 1 as Lantiq does?

Thanks,
Stefan