[src/trunk]: src/sys/arch/arm/cortex Improve delay to be better than 1.3 inna...

[skrll <skrll%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/7bd6e1ab9154
branches:  trunk
changeset: 827373:7bd6e1ab9154
user:      skrll <skrll%NetBSD.org@localhost>
date:      Wed Oct 25 16:09:46 2017 +0000

description:
Improve delay to be better than 1.3 innaccurate by default and handle the
unlikely wrap

diffstat:

 sys/arch/arm/cortex/gtmr.c |  30 ++++++++++++++++++------------
 1 files changed, 18 insertions(+), 12 deletions(-)

diffs (54 lines):

diff -r 7edbf6083931 -r 7bd6e1ab9154 sys/arch/arm/cortex/gtmr.c
--- a/sys/arch/arm/cortex/gtmr.c        Wed Oct 25 16:08:09 2017 +0000
+++ b/sys/arch/arm/cortex/gtmr.c        Wed Oct 25 16:09:46 2017 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: gtmr.c,v 1.21 2017/10/25 16:08:09 skrll Exp $  */
+/*     $NetBSD: gtmr.c,v 1.22 2017/10/25 16:09:46 skrll Exp $  */
 
 /*-
  * Copyright (c) 2012 The NetBSD Foundation, Inc.
@@ -30,7 +30,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: gtmr.c,v 1.21 2017/10/25 16:08:09 skrll Exp $");
+__KERNEL_RCSID(0, "$NetBSD: gtmr.c,v 1.22 2017/10/25 16:09:46 skrll Exp $");
 
 #include <sys/param.h>
 #include <sys/bus.h>
@@ -245,19 +245,25 @@
        uint32_t freq = sc->sc_freq ? sc->sc_freq : armreg_cnt_frq_read();
        KASSERT(freq != 0);
 
-       /*
-        * not quite divide by 1000000 but close enough
-        * (higher by 1.3% which means we wait 1.3% longer).
-        */
-       const uint64_t incr_per_us = (freq >> 20) + (freq >> 24);
+       const unsigned int incr_per_us = howmany(freq, 1000000);
+       unsigned int delta = 0, usecs = 0;
 
        arm_isb();
-       const uint64_t base = armreg_cntp_ct_read();
-       const uint64_t delta = n * incr_per_us;
-       const uint64_t finish = base + delta;
+       uint64_t last = armreg_cntp_ct_read();
 
-       while (armreg_cntp_ct_read() < finish) {
-               arm_isb();      /* spin */
+       while (n > usecs) {
+               arm_isb();
+               uint64_t curr = armreg_cntp_ct_read();
+               if (curr < last)
+                       delta += curr + (UINT64_MAX - last);
+               else
+                       delta += curr - last;
+
+               last = curr;
+               if (delta >= incr_per_us) {
+                       usecs += delta / incr_per_us;
+                       delta %= incr_per_us;
+               }
        }
 }