U-Boot runs in supervisor mode. On ARMv6 and lower, software interrupts
are taken in supervisor mode. When entering an interrupt, the link
register is set to the address of the next instruction. However, if we
are already in supervisor mode, this clobbers the link register. The
debugger can't help us, since by the time it notices we've taken a
software interrupt, the link register is already gone. Work around this
by moving the return address to another register.
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
So far we used inline assembly to inject the actual instruction that
triggers the semihosting service. While this sounds elegant, as it's
really only about one instruction, it has some serious downsides:
- We need some barriers in place to force the compiler to issue writes
to a data structure before issuing the trap instruction.
- We need to convince the compiler to actually fill the structures that
we use pointers to.
- We need a memory clobber to avoid the compiler caching the data in
those structures, when semihosting writes data back.
- We need register arguments to make sure the function ID and the
pointer land in the right registers.
This is all doable, but fragile and somewhat cumbersome. Since we now
have a separate function in an extra file anyway, we can do away with
all the magic and just write that in an actual assembly file.
This is much more readable and robust.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
