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

[Philippe Mathieu-Daudé]

On 3/10/21 2:10 PM, Laurent Vivier wrote:
> 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...

Good candidate to switch to the Clock API, see docs/devel/clocks.rst
once this gets merged:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg788864.html

> 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
> 
>