On Fri, Sep 02, 2022 at 11:42:03AM -0500, Scott Cheloha wrote:
> 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
Didn't see dv's reply earlier; I agree with what he said.
>
> 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;
