[U-Boot] [RFC] Review of U-Boot timer API

[Reinhard Meyer <u-boot@emk-elektronik.de>]

I know its futile to repeat what I suggested about 9 months ago...

Since get_timer() is only used (to my knowledge) to break out of
loops that do not terminate normally because an expected event does
not occur, the following API would be simpler and take less time per
loop:

(I use the names A and B to avoid nitpicking on real function names
which can be later chosen arbitrarily if this idea prevails...)

uint timeout = A(timeout_in_ms);

do {...} while (!B(timeout));

OR

for (;;) {
   dowhatever...
   if (B(timeout)) {
     error_handling...
     break;
   }
}

Internally both functions would be rather trival:

uint A(uint timeout_in_ms):

return current_tick + timeout_in_ms * ticks_per_ms;

or - for better precision if ticks_per_ms is not a whole number:

return current_tick + (timeout_in_ms * ticks_per_second) / 1000;

or any fractional method to more exactly calculate that equation
without needing a 64 bit divide.

bool B(uint end_tick):

a simple unsigned subtraction of end_tick minus current_tick,
evaluating the carry flag. Since we don't get a carry flag in C
a simple methods will do:
return ((int)(end_tick - current_tick)) < 0;

options:
current_tick needs only to be uint32, to get to the largest possible timeout
any 64 bit tick counter can be right shifted to get a 32 bit value that
increments just faster than 1ms.

features:
the only complicated calculation is done before the loop starts, the loop
itself is not made significantly slower since only a subtraction (and maybe
shift to scale down a high speed tick) is required.

Reinhard