Switching the computation of cycles/delaycnt to a proper 64 value math
instead of the '32 bit safe' complex math is likely a good idea.
However I am not completely convinced that switching to 'yield' (current
CPU_BUSY_CYCLE implementation) for every loop of a 'short wait' in the wait
loop makes sense. In a hypervisor environment, this could cause a very
short wait between register writes to become very long, In a non-hypervisor
environment there is essentially no improvement because the yield doesn't
really have any benefits on non-hypervisor.
To my current understanding, there is no useful 'wait short period' on arm
cores.
On Mon, Aug 7, 2023 at 10:32 PM Scott Cheloha <scottchel...@gmail.com>
wrote:
> The agtimer(4/arm64) delay(9) implementation is quite complicated.
> This patch simplifies it.
>
> Am I missing something here? There's no reason to implement the
> busy-loop like this, right?
>
> ok?
>
> Index: agtimer.c
> ===================================================================
> RCS file: /cvs/src/sys/arch/arm64/dev/agtimer.c,v
> retrieving revision 1.23
> diff -u -p -r1.23 agtimer.c
> --- agtimer.c 25 Jul 2023 18:16:19 -0000 1.23
> +++ agtimer.c 8 Aug 2023 02:24:57 -0000
> @@ -323,32 +323,12 @@ agtimer_cpu_initclocks(void)
> void
> agtimer_delay(u_int usecs)
> {
> - uint64_t clock, oclock, delta, delaycnt;
> - uint64_t csec, usec;
> - volatile int j;
> + uint64_t cycles, start;
>
> - if (usecs > (0x80000000 / agtimer_frequency)) {
> - csec = usecs / 10000;
> - usec = usecs % 10000;
> -
> - delaycnt = (agtimer_frequency / 100) * csec +
> - (agtimer_frequency / 100) * usec / 10000;
> - } else {
> - delaycnt = agtimer_frequency * usecs / 1000000;
> - }
> - if (delaycnt <= 1)
> - for (j = 100; j > 0; j--)
> - ;
> -
> - oclock = agtimer_readcnt64();
> - while (1) {
> - for (j = 100; j > 0; j--)
> - ;
> - clock = agtimer_readcnt64();
> - delta = clock - oclock;
> - if (delta > delaycnt)
> - break;
> - }
> + start = agtimer_readcnt64();
> + cycles = (uint64_t)usecs * agtimer_frequency / 1000000;
> + while (agtimer_readcnt64() - start < cycles)
> + CPU_BUSY_CYCLE();
> }
>
> void
>
>
