[U-Boot] [PATCH 2/9] MIPS: fix ROM exception vectors

[Matthew Fortune]

Daniel Schwierzeck <daniel.schwierzeck at gmail.com> writes:
> When booting from ROM, early exceptions can't be handled
> properly. Instead of busy-looping give the developer the
> possibilty to examine the situation. Thus issue a SDBBP
> instruction to transfer control to hardware debugger if one
> is attached.

You could make the SDBBP into a UHI operation that can be read by
a debugger as an unhandled exception rather than an unexpected
breakpoint (assuming said debugger knows about UHI). The fragment
I use in lightweight boot code is:

    move  k0, t9                        # Preserve t9
    move  k1, a0                        # Preserve a0
    li    $25, 15                       # UHI exception operation
    li    $4, 0                         # Use hard register context
    sdbbp 1                             # Invoke UHI operation

You lose k0/k1 in this which may be undesirable but it might
be a reasonable trade off. This shouldn't go in the debug vector
of course!

Matthew