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

[Laurent Vivier]

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

Thanks,
Laurent