[ECOS] Redboot reset must use JUMP ?

[Jordi Colomer <jco at ict dot es>]

Hi,

(Thanks to Gary Thomas for replying my first question
about this problem).

I have tried redboot for the first time in my Assabet
(StrongArm) board. After writing it on the flash memory,
it works great : I can use the network, download a kernel
image and flash it. Doing a hardware reset also works ok,
as well.

After recycling power (power off and then on), the CPU 
does not boot. Sampling the logic levels of the pins
with a boundary-scan utility, reveals that the CPU is 
constantly receiving prefetch aborts. Since the MMU is
disabled, the CPU cannot handle the *virtual* jump address 
to the prefetch handler, then a new prefetch abort is
thrown and loops back again.

It seems that the first prefetch abort is raised right
after the CPU executes the reset vector, which contains
an indirect jump to the reset routine. This routine never
executes because the exception is thrown, making the CPU
to enter the endless loop.

If I define CYGSEM_HAL_ROM_RESET_USES_JUMP in 
hal/arm/arch/src/vectors.S, then a direct jump is used,
and the prefetch abort is NOT thrown. Redboot boots ok.


I wonder :

1. Why the indirect jump causes a prefetch abort, while
the MMU is disabled ? (a chip bug... ?). It does not
happen in some other assabets.

2. Why the code in vectors.S is conditional at this 
point ? which are the benefits of the indirect jump ?
is it really necessary (although legal) ?

This is part of hal/arm/.../vectors.S :

.global __exception_handlers
__exception_handlers:
#ifdef CYGSEM_HAL_ROM_RESET_USES_JUMP
// Assumption:  ROM code has these vectors at the hardware reset
address.
// A simple jump removes any address-space dependencies [i.e. safer]
        b       reset_vector                    // 0x00
#else
        ldr     pc,.reset_vector                // 0x00
#endif
        ldr     pc,.undefined_instruction       // 0x04
        ldr     pc,.software_interrupt          // 0x08 start &&
software int
        ldr     pc,.abort_prefetch              // 0x0C
        ldr     pc,.abort_data                  // 0x10
        .word   0                               // unused
        ldr     pc,.IRQ                         // 0x18
        ldr     pc,.FIQ                         // 0x1C


NOTE: .reset_vector contains a physical address. The other variables
contain virtual addresses.


Jordi Colomer.