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

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(-)

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...

> @@ -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.

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()? I'm guessing 
that Linux/m68k does use one or both of the timers in one-shot mode?

> @@ -279,6 +285,7 @@ void mos6522_write(void *opaque, hwaddr addr, uint64_t val, unsigned size)
>   {
>       MOS6522State *s = opaque;
>       MOS6522DeviceClass *mdc = MOS6522_GET_CLASS(s);
> +    int64_t now;
>   
>       trace_mos6522_write(addr, val);
>   
> @@ -318,9 +325,6 @@ void mos6522_write(void *opaque, hwaddr addr, uint64_t val, unsigned size)
>           s->timers[1].latch = (s->timers[1].latch & 0xff00) | val;
>           break;
>       case VIA_REG_T2CH:
> -        /* To ensure T2 generates an interrupt on zero crossing with the
> -           common timer code, write the value directly from the latch to
> -           the counter */
>           s->timers[1].latch = (s->timers[1].latch & 0xff) | (val << 8);
>           s->ifr &= ~T2_INT;
>           set_counter(s, &s->timers[1], s->timers[1].latch);
> @@ -330,8 +334,9 @@ void mos6522_write(void *opaque, hwaddr addr, uint64_t val, unsigned size)
>           break;
>       case VIA_REG_ACR:
>           s->acr = val;
> -        mos6522_timer1_update(s, &s->timers[0],
> -                              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
> +        now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +        mos6522_timer1_update(s, &s->timers[0], now);
> +        mos6522_timer2_update(s, &s->timers[1], now);
>           break;
>       case VIA_REG_PCR:
>           s->pcr = val;
> diff --git a/include/hw/misc/mos6522.h b/include/hw/misc/mos6522.h
> index fc95d22b0f..94b1dc324c 100644
> --- a/include/hw/misc/mos6522.h
> +++ b/include/hw/misc/mos6522.h
> @@ -50,8 +50,10 @@
>   #define T1_INT             0x40    /* Timer 1 interrupt */
>   
>   /* Bits in ACR */
> +#define T2MODE             0x20    /* Timer 2 mode */
>   #define T1MODE             0xc0    /* Timer 1 mode */
>   #define T1MODE_CONT        0x40    /*  continuous interrupts */
> +#define T1MODE_ONESHOT     0x00    /*  timed interrupt */
>   
>   /* VIA registers */
>   #define VIA_REG_B       0x00
> @@ -83,6 +85,7 @@ typedef struct MOS6522Timer {
>       int64_t next_irq_time;
>       uint64_t frequency;
>       QEMUTimer *timer;
> +    bool oneshot_fired;
>   } MOS6522Timer;
>   
>   /**


ATB,

Mark.

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