The 8254 data sheet [1] says this about the Read-Back command:
> The read-back command may be used to latch multi-
> ple counter output latches (OL) by setting the
> COUNT bit D5 = 0 and selecting the desired coun-
> ter(s). This single command is functionally equiva-
> lent to several counter latch commands, one for
> each counter latched. [...]
This is a little ambiguous. But my hunch is that the intent here is
"you can latch multiple counters all at once". Simultaneously.
Otherwise the utility of the read-back command is suspect.
To simulate a simultaneous latch, we should only call clock_gettime(2)
once and use that singular timestamp to compute olatch for each
counter.
ok?
[1] 8254 Programmable Interval Timer, p. 8
https://www.scs.stanford.edu/10wi-cs140/pintos/specs/8254.pdf
Index: i8253.c
===================================================================
RCS file: /cvs/src/usr.sbin/vmd/i8253.c,v
retrieving revision 1.34
diff -u -p -r1.34 i8253.c
--- i8253.c 16 Jun 2021 16:55:02 -0000 1.34
+++ i8253.c 2 Sep 2022 16:25:02 -0000
@@ -128,6 +128,8 @@ i8253_do_readback(uint32_t data)
int readback_channel[3] = { TIMER_RB_C0, TIMER_RB_C1, TIMER_RB_C2 };
int i;
+ clock_gettime(CLOCK_MONOTONIC, &now);
+
/* bits are inverted here - !TIMER_RB_STATUS == enable chan readback */
if (data & ~TIMER_RB_STATUS) {
i8253_channel[0].rbs = (data & TIMER_RB_C0) ? 1 : 0;
@@ -139,7 +141,6 @@ i8253_do_readback(uint32_t data)
if (data & ~TIMER_RB_COUNT) {
for (i = 0; i < 3; i++) {
if (data & readback_channel[i]) {
- clock_gettime(CLOCK_MONOTONIC, &now);
timespecsub(&now, &i8253_channel[i].ts, &delta);
ns = delta.tv_sec * 1000000000 + delta.tv_nsec;
ticks = ns / NS_PER_TICK;
