For one thing, microblaze does *not* have kernel_stack_pointer(),
so defaults would not work there. That leads to build breakage in
kernel/trace/trace_function.c since the commit in question. Breakage is
trivial to reproduce - just enable FUNCTION_GRAPH_TRACER and try to
build the kernel...
Moreover, unless I'm misreading their mcount.S, the registers saved
there are *NOT* in their pt_regs layout - the stored data is
r15 (return address of _mcount())
r2..r14
r16..r31
and all of that is at r1 - 120, where r1 is the kernel stack pointer at the
time of _mcount() call. So those defaults would be all wrong anyway.
The last time I touched microblaze assembler had been a dozen years
ago, so I'd rather avoid trying to patch the things up there - not with
live microblaze maintainers around, TYVM...
