interesting line in process.c

I'm intrigued by this line, line 276 in arch/ppc/kernel/process.c in
linuxppc_2_4_devel:

#if defined(CONFIG_4xx) && defined(DCRN_PLB0_BEAR) && defined(DCRN_PLB0_BEAR)

When could we have DCRN_PLB0_BEAR defined but DCRN_PLB0_BEAR not? :)
Could it ever be defined if CONFIG_4xx was not defined?

Which brings up another question that I have been meaning to ask: what
is the rationale for adding the dbcr0/1 fields to the ptrace struct
for 4xx?

Since struct ptrace is part of the kernel/user ABI, I prefer not to
change it unless it is absolutely necessary.  Could the dbcr0/1 fields
go in the thread_struct instead?  Where and how are they used?

Paul.

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