Re: [PATCH v2 6/7] mac_via: fix 60Hz VIA1 timer interval

[BALATON Zoltan]

On Thu, 11 Mar 2021, Mark Cave-Ayland wrote:
> The 60Hz timer is initialised using timer_new_ns() meaning that the timer
> interval should be measured in ns, and therefore its period is a thousand
> times too short.
>
> Use a define for the 60Hz timer period taking the more precise value as
> documented in the Guide To The Macintosh Family Hardware.
>
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> Reviewed-by: Laurent Vivier <laurent@vivier.eu>

Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>

> ---
> hw/misc/mac_via.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/hw/misc/mac_via.c b/hw/misc/mac_via.c
> index f994fefa7c..e7dd31f291 100644
> --- a/hw/misc/mac_via.c
> +++ b/hw/misc/mac_via.c
> @@ -279,6 +279,12 @@
> #define VIA_TIMER_FREQ (783360)
> #define VIA_ADB_POLL_FREQ 50 /* XXX: not real */
>
> +/*
> + * Guide to the Macintosh Family Hardware ch. 12 "Displays" p. 401 gives the
> + * precise 60Hz interrupt frequency as ~60.15Hz with a period of 16625.8 us
> + */
> +#define VIA_60HZ_TIMER_PERIOD_NS   16625800
> +
> /* VIA returns time offset from Jan 1, 1904, not 1970 */
> #define RTC_OFFSET 2082844800
>
> @@ -302,8 +308,9 @@ static void via1_sixty_hz_update(MOS6522Q800VIA1State *v1s)
>     MOS6522State *s = MOS6522(v1s);
>
>     /* 60 Hz irq */
> -    v1s->next_sixty_hz = (qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + 16630) /
> -                          16630 * 16630;
> +    v1s->next_sixty_hz = (qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +
> +                          VIA_60HZ_TIMER_PERIOD_NS) /
> +                          VIA_60HZ_TIMER_PERIOD_NS * VIA_60HZ_TIMER_PERIOD_NS;
>
>     if (s->ier & VIA1_IRQ_60HZ) {
>         timer_mod(v1s->sixty_hz_timer, v1s->next_sixty_hz);