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Re: [RFC 06/10] hw/mos6522: Implement oneshot mode
From: |
Finn Thain |
Subject: |
Re: [RFC 06/10] hw/mos6522: Implement oneshot mode |
Date: |
Sun, 29 Aug 2021 11:20:46 +1000 (AEST) |
On Wed, 25 Aug 2021, Mark Cave-Ayland wrote:
> On 24/08/2021 11:09, Finn Thain wrote:
>
> > Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> > ---
> > hw/misc/mos6522.c | 19 ++++++++++++-------
> > include/hw/misc/mos6522.h | 3 +++
> > 2 files changed, 15 insertions(+), 7 deletions(-)
> >
> > diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
> > index ffff8991f4..5b1657ac0d 100644
> > --- a/hw/misc/mos6522.c
> > +++ b/hw/misc/mos6522.c
> > @@ -79,6 +79,7 @@ static void set_counter(MOS6522State *s, MOS6522Timer *ti,
> > unsigned int val)
> > trace_mos6522_set_counter(1 + ti->index, val);
> > ti->load_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> > ti->counter_value = val;
> > + ti->oneshot_fired = false;
> > if (ti->index == 0) {
> > mos6522_timer1_update(s, ti, ti->load_time);
> > } else {
> > @@ -133,7 +134,8 @@ static void mos6522_timer1_update(MOS6522State *s,
> > MOS6522Timer *ti,
> > return;
> > }
> > ti->next_irq_time = get_next_irq_time(s, ti, current_time);
> > - if ((s->ier & T1_INT) == 0 || (s->acr & T1MODE) != T1MODE_CONT) {
> > + if ((s->ier & T1_INT) == 0 ||
> > + ((s->acr & T1MODE) == T1MODE_ONESHOT && ti->oneshot_fired)) {
> > timer_del(ti->timer);
> > } else {
> > timer_mod(ti->timer, ti->next_irq_time);
> > @@ -147,7 +149,7 @@ static void mos6522_timer2_update(MOS6522State *s,
> > MOS6522Timer *ti,
> > return;
> > }
> > ti->next_irq_time = get_next_irq_time(s, ti, current_time);
> > - if ((s->ier & T2_INT) == 0) {
> > + if ((s->ier & T2_INT) == 0 || (s->acr & T2MODE) || ti->oneshot_fired) {
> > timer_del(ti->timer);
> > } else {
> > timer_mod(ti->timer, ti->next_irq_time);
> > @@ -159,6 +161,7 @@ static void mos6522_timer1_expired(void *opaque)
> > MOS6522State *s = opaque;
> > MOS6522Timer *ti = &s->timers[0];
> > + ti->oneshot_fired = true;
> > mos6522_timer1_update(s, ti, ti->next_irq_time);
> > s->ifr |= T1_INT;
> > mos6522_update_irq(s);
> > @@ -169,6 +172,7 @@ static void mos6522_timer2_expired(void *opaque)
> > MOS6522State *s = opaque;
> > MOS6522Timer *ti = &s->timers[1];
> > + ti->oneshot_fired = true;
> > mos6522_timer2_update(s, ti, ti->next_irq_time);
> > s->ifr |= T2_INT;
> > mos6522_update_irq(s);
>
> I was trying to understand why you need ti->oneshot_fired here since the
> mos6522_timer*_update() functions should simply not re-arm the timer if
> not in continuous mode...
>
Not so. The timer has to be re-armed with timer_mod() when
(timer interrupt enabled and timer in continuous mode) ||
(timer interrupt enabled and timer in oneshot mode and no interrupt raised)
Conversely, the timer has to be cancelled with timer_del() when
(timer interrupt disabled) ||
(timer in oneshot mode and interrupt has been raised) ||
(timer in pulse-counting mode)
> > @@ -198,10 +202,12 @@ uint64_t mos6522_read(void *opaque, hwaddr addr,
> > unsigned size)
> > int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> > if (now >= s->timers[0].next_irq_time) {
> > + s->timers[0].oneshot_fired = true;
> > mos6522_timer1_update(s, &s->timers[0], now);
> > s->ifr |= T1_INT;
> > }
> > if (now >= s->timers[1].next_irq_time) {
> > + s->timers[1].oneshot_fired = true;
> > mos6522_timer2_update(s, &s->timers[1], now);
> > s->ifr |= T2_INT;
> > }
>
> ...however this block above raises the timer interrupt outside of the
> timer callback. This block isn't part of your original patch but was
> introduced as part of cd8843ff25d ("mos6522: fix T1 and T2 timers") but
> I'm wondering if it is wrong.
>
Maybe. I think a good answer would make reference to QEMU internals and
synchronization guarantees between the invocation of the callbacks and
methods in mos6522.c. I don't have a good answer, but it's moot...
> If you remove both of the above if (now ... ) {} blocks then does
> one-shot mode work by just adding the (s->acr & T2MODE) check in
> mos6522_timer2_update()?
>
Those blocks got removed in patch 10/10 because they aren't needed as long
as get_counter() gets called when necessary.
> I'm guessing that Linux/m68k does use one or both of the timers in
> one-shot mode?
>
Yes, but it's not in mainline yet. I wrote the code some months ago but
I can't push it upstream until QEMU supports it:
https://github.com/fthain/linux/commits/clockevent-oneshot
- Re: [RFC 07/10] hw/mos6522: Fix initial timer counter reload, (continued)
- [RFC 03/10] hw/mos6522: Remove redundant mos6522_timer1_update() calls, Finn Thain, 2021/08/24
- [RFC 10/10] hw/mos6522: Synchronize timer interrupt and timer counter, Finn Thain, 2021/08/24
- [RFC 02/10] hw/mos6522: Remove get_counter_value() methods and functions, Finn Thain, 2021/08/24
- [RFC 06/10] hw/mos6522: Implement oneshot mode, Finn Thain, 2021/08/24
- [RFC 09/10] hw/mos6522: Avoid using discrepant QEMU clock values, Finn Thain, 2021/08/24
- Re: [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements, David Gibson, 2021/08/25
- Re: [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements, Mark Cave-Ayland, 2021/08/25