[src/trunk]: src/sys/dev/ic On attach figure out how long a single read of th...

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

details:   https://anonhg.NetBSD.org/src/rev/6a38b433ada2
branches:  trunk
changeset: 931241:6a38b433ada2
user:      ad <ad%NetBSD.org@localhost>
date:      Fri Apr 24 22:25:07 2020 +0000

description:
On attach figure out how long a single read of the counter register takes
and use that for the adjustment in hpet_delay().

diffstat:

 sys/dev/ic/hpet.c |  27 ++++++++++++++++++++-------
 1 files changed, 20 insertions(+), 7 deletions(-)

diffs (65 lines):

diff -r cd54640c67e9 -r 6a38b433ada2 sys/dev/ic/hpet.c
--- a/sys/dev/ic/hpet.c Fri Apr 24 22:07:12 2020 +0000
+++ b/sys/dev/ic/hpet.c Fri Apr 24 22:25:07 2020 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: hpet.c,v 1.14 2020/04/23 20:33:57 ad Exp $ */
+/* $NetBSD: hpet.c,v 1.15 2020/04/24 22:25:07 ad Exp $ */
 
 /*
  * Copyright (c) 2006 Nicolas Joly
@@ -33,7 +33,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: hpet.c,v 1.14 2020/04/23 20:33:57 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: hpet.c,v 1.15 2020/04/24 22:25:07 ad Exp $");
 
 #include <sys/systm.h>
 #include <sys/device.h>
@@ -79,7 +79,7 @@
        struct hpet_softc *sc = device_private(dv);
        struct timecounter *tc;
        uint64_t tmp;
-       uint32_t val;
+       uint32_t val, sval, eval;
        int i;
 
        tc = &sc->sc_tc;
@@ -130,6 +130,19 @@
 
        if (device_unit(dv) == 0)
                hpet0 = sc;
+
+       /*
+        * Determine approximately how long it takes to read the counter
+        * register once, and compute an ajustment for hpet_delay() based on
+        * that.
+        */
+       (void)bus_space_read_4(sc->sc_memt, sc->sc_memh, HPET_MCOUNT_LO);
+       sval = bus_space_read_4(sc->sc_memt, sc->sc_memh, HPET_MCOUNT_LO);
+       for (i = 0; i < 998; i++)
+               (void)bus_space_read_4(sc->sc_memt, sc->sc_memh, HPET_MCOUNT_LO);
+       eval = bus_space_read_4(sc->sc_memt, sc->sc_memh, HPET_MCOUNT_LO);
+       val = eval - sval;
+       sc->sc_adj = (int64_t)val * sc->sc_period / 1000;
 }
 
 static u_int
@@ -168,13 +181,13 @@
        int64_t delta;
 
        /*
-        * Read timer before slow division.  Assume that each read of the
-        * HPET costs ~500ns.  Aim for the middle and subtract 750ns for
-        * overhead.
+        * Read timer before slow division.  Convert microseconds to
+        * femtoseconds, subtract the cost of 1 counter register access,
+        * and convert to HPET units.
         */
        sc = hpet0;
        otick = bus_space_read_4(sc->sc_memt, sc->sc_memh, HPET_MCOUNT_LO);
-       delta = (((int64_t)us * 1000000000) - 750000000) / sc->sc_period;
+       delta = (((int64_t)us * 1000000000) - sc->sc_adj) / sc->sc_period;
 
        while (delta > 0) {
                SPINLOCK_BACKOFF_HOOK;