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

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 10/03/2021 13:24, Laurent Vivier wrote:

> Le 10/03/2021 à 14:10, Laurent Vivier a écrit :
>> Le 10/03/2021 à 13:56, Laurent Vivier a écrit :
>>> Le 10/03/2021 à 13:32, BALATON Zoltan a écrit :
>>>> On Wed, 10 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.
>>>>>
>>>>> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
>>>>> ---
>>>>> hw/misc/mac_via.c | 4 ++--
>>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/hw/misc/mac_via.c b/hw/misc/mac_via.c
>>>>> index f994fefa7c..c6e1552a59 100644
>>>>> --- a/hw/misc/mac_via.c
>>>>> +++ b/hw/misc/mac_via.c
>>>>> @@ -302,8 +302,8 @@ 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) + 16630000) /
>>>>> +                          16630000 * 16630000;
>>>>
>>>> Can you put this magic number in a #define maybe also rewriting it in a way that shows it
>>>> corresponds to a 60 Hz interval. (There's NANOSECONDS_PER_SECOND defined in include/qemu/timer.h
>>>> that could be used for that, there's also SCALE_MS that might replace 1000 * 1000 elsewhere in this
>>>> file). Also NANOSECONDS_PER_SECOND / 60 is 16666666, should that value be used here instead?
>>>
>>> In fact, the Mac Frequency is not exactly 60 Hz, in docs we can find 60.147 Hz, in kernel 60.15 Hz.
>>> I Think there are several ways to compute it...
>>>
>>
>> In fact, we can read:
>>
>> "the vertical retrace frequency is approximately 60.15 Hz, resulting in a period of approximately
>> 16.63 milliseconds"
>>
>> https://developer.apple.com/library/archive/documentation/mac/pdf/Processes/Vertical_Retrace_Mgr.pdf
> 
> The exact value is 16625800 ns
> 
> "Macintosh Family Hardware Reference" ISBN 0-201-19255-1
> "The video interface"
> p. 13-3
> 
> "[...] This means the full frame is redisplayed every 370 scan lines, or once every 166625.8 µs."

Thanks Laurent! Given that the exact precision is 6 digits I don't think it's 
possible to make use of conversion macros without either making it harder to read or 
reducing the precision.

I think the best solution here would be to #define VIA1_60HZ_TIMER_PERIOD_NS with a 
comment containing the above reference, and use that in the period calculation. Would 
that be sufficient?


ATB,

Mark.