agtimer(4/arm64): agtimer_delay: compute cycle count with 64-bit arithmetic

Converting from microseconds to timer cycles is much simpler with
64-bit arithmetic.

Thread: https://marc.info/?l=openbsd-tech&m=169146193022516&w=2

ok drahn@ kettenis@

diff --git a/sys/arch/arm64/dev/agtimer.c b/sys/arch/arm64/dev/agtimer.c
index 08e93b4..ffe40b2 100644 (file)
--- a/sys/arch/arm64/dev/agtimer.c
+++ b/sys/arch/arm64/dev/agtimer.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: agtimer.c,v 1.23 2023/07/25 18:16:19 cheloha Exp $ */
+/* $OpenBSD: agtimer.c,v 1.24 2023/08/10 22:58:04 cheloha Exp $ */
 /*
  * Copyright (c) 2011 Dale Rahn <drahn@openbsd.org>
  * Copyright (c) 2013 Patrick Wildt <patrick@blueri.se>
@@ -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;
-
-       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;
-       }
+       uint64_t cycles, start;
+
+       start = agtimer_readcnt64();
+       cycles = (uint64_t)usecs * agtimer_frequency / 1000000;
+       while (agtimer_readcnt64() - start < cycles)
+               continue;
 }
 
 void