Global data (pointed by R2 on PowerPC) in principle is not guaranteed to be in
proximity of U-Boot heap (where the API signature is placed) accross different
architectures and platforms. Instead, use U-Boot stack pointer as a hint for
the search instead of the global data; this method tends to be more uniform
accross different platforms.
Obtained from: Semihalf
o Disable interrupts while not running U-Boot code. We clobber
registers that the U-Boot interrupt handlers assume to be
fixed as per the U-Boot register usage. At this time this only
applies to r14. U-Boot uses r2 now for what they used r29 for.
After we restore r14 in preparation of doing the syscall, we
re-enable interrupts. When we return from the syscall, we
disable interrupts and restore the callee-saved r14.
