[U-Boot] driver model is not smp safe

[Bin Meng]

Hi Simon,

When adding x86 multi-cpu initialization on a board with 4 cores, I found:

=> cpu list
  0: cpu at 0               Genuine Intel(R) CPU         @ 1.58GHz
  1: cpu at 1               Genuine Intel(R) CPU         @ 1.58GHz
  2: cpu at 2               Genuine Intel(R) CPU         @ 1.58GHz
  2: cpu at 3               Genuine Intel(R) CPU         @ 1.58GHz

cpu at 2 and cpu at 3 have the same sequence number, which indicates they
are running parallelly to get the same sequence number. The call chain
on an ap is: mp_init_cpu() -> device_probe() -> uclass_resolve_seq().
Apparently ap2 and ap3 are running at the same time to get the same
number.

Note so far all x86 boards that we have enabled x86 multi-cpu
initialization on only have 2 cores, which will not expose such issue
as there is no parallel execution among aps.

What does this mean?

-  Driver model is not smp safe. But given U-Boot is a single-threaded
environment, I don't think we want to add such support to driver
model.

OR:

- We are using driver model incorrectly on x86 mp initialization codes.

What do you think?

Regards,
Bin