Re: [RFC 06/10] hw/mos6522: Implement oneshot mode

[Finn Thain <fthain@linux-m68k.org>]

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