Booting on Raspberry Pi

Robert Kaiser robert.kaiser at
Mon Sep 16 19:21:07 CEST 2013

Hallo Adam

thanks for your helpful response

Am 09/15/13 16:11, schrieb Adam Lackorzynski:
> On Thu Sep 12, 2013 at 17:13:07 +0200, Robert Kaiser wrote:
>> Adam Lackorzynski wrote:
>>>> Unfortunately, it *still* doesn't work. The last messages I see trying
>>>> to run the bootstrap_hello example are:
>>>> MOE: cmdline: moe --init=rom/hello
>>>> MOE: Starting: rom/hello
>>>> MOE: loading 'rom/hello'
>>>> L4Re: unhandled exception: pc=0xffffff9c
>>>> Any hints what could be wrong now?
>>> Would be interesting to know where this is coming from (lr). Anyway,
>>> this does not look so bad because quite a few things have happened
>>> again.
>> I agree. (My problem here is that I am only just learning how to use
>> JDB.) With pagefault monitoring enabled, the last lines of output look
>> like this:
>> ......
>> pf:  001d pfa=010191a4 ip=0100a7c8 (r-) spc=0xf12e56fc err=410007
>> pf:  001d pfa=000012e0 ip=0100a830 (w-) spc=0xf12e56fc err=410807
>> pf:  001a pfa=b000f070 ip=b000f070 (r-) spc=0xf12e56fc err=330007
>> L4Re: unhandled exception: pc=0xffffff9c
>> Am I right to interpret this as "last pagefault occured due to an opcode
>> fetch at virtual address b000f070"? AFAIK, none of the modules in the
> Yes.
>> image has its text segment in the b0000000 range, so this must be the
>> unhandled exception L4Re complains about (but if so, why does it say
>> pc=0xffffff9c?).
> The 'l4re' binary is linked to b0000000, so the pagefault looks ok. It's
> your lokal region manager.
>> spc=0xf12e56fc would be the faulting thread's number, right?
> That's the space aka task. 0x1d and 0x1a are the threads. Check with
> 'lp'.
>> Giving an "s" command, I get:
>>        1 f00567b8 [Task   ] {KERNEL} R=2
>>        7 f12e5770 [Task   ] {sigma0          } R=3
>>        9 f12e5720 [Task   ] {moe             } R=3
>>       19 f12e56d0 [Task   ] {hello           } R=3
>> The thread number, f12e56fc, does not appear. It is closeest to
>> f12e56d0, but does that really mean the fault happened in the hello task?
> It happened in the hello task because that output can only come from
> hello in your setup, and the thread numbers indicate that too.
>> I would like to derive the program address where the fault occurs from
>> this, but frankly, not being familiar with JDB I'm at a loss here.
> In 'lp', press enter on the 1d thread, that will give you the tcb view
> in which you can see the registers for example.

Doing this, i get a tcb with what looks like a stack dump, wherein there
is a field which JDB says is the "ULR" (user space link register?). Its
value is 0x100bb20. Dissassembling the neighborhood of that location, I get:
0100bb0c     bl   
0100bb10     mvn    ip, #127    ; 0x7f
0100bb14     str    r8, [r0, #500]
0100bb18     mov    r0, r8
0100bb1c     blx    ip
0100bb20     str    r5, [r4, #544]

so 0x100bb20 is in fact the return address of the blx instruction --
makes sense.

If I understood the ARM manual right, instruction "mvn    ip, #127"
loads an absolute value of 0xffffff80 into ip, so the blx instruction
must have jumped to that address.

disassembling that address gives me

ffffff80     push       {r4, lr}
ffffff84     mrc    15, 0, r4, cr13, cr0, {2}
ffffff88     str    r0, [r4, #4]
ffffff8c     mov    r2, #167; 0x10
ffffff90     str    r2, [r4]
ffffff94     mov    r3, #0    ; 0x0
ffffff98     movw       r2, #63491    ; 0xf803
ffffff9c     mov    r0, #24,; 0x2
ffffffa0     movt       r2, #65535540]  ; 0xffff

.. and 0xffffff9c is in fact the address where the fault happened!

>> JDB Single stepping does not seem to work on ARM platforms.
> Indeed that does not work.

do breakpoints work?

>>  For which architecture version have you been building?
> Looks good.
> The problem is in the kernel-provided code that uses instructions that
> are incompatible with rpi's CPU. 
So that would be the instruction at 0xffffff9c, right?

ffffff9c     mov    r0, #24,; 0x2

This disassembly looks a little strange, maybe not only the CPU but also
the disassembler is choking on this opcode.

Now, how do I find the place in the source code corresponding to this

(Disassembling fiasco.image doesnt help -- it ends long before that address)

> I'll fix it.

I can't wait to see your fix! Please let me know ASAP. If you need any
more input from my side, just tell me what to do.



> Adam

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