On Tuesday 15 September 2015 09:48:10 Pingbo Wen wrote:
> >
> >> - does the driver use monotonic or real time
> >> Real time. But only microsecond part is used.
> >>
> >> - if it uses real time, should it use monotonic time instead
> >> Monotonic time will be ok if it can meet the precise requirement(us).
> >
> > Your patch picks the easy approach by leaving the driver to use real
> > time. This clearly has a lower risk of regressions, which is good, but
> > you should come up with an argument on which of the two is the better
> > choice here.
> >
>
> Yes, I just follow the old way to avoid additional risks. If using monotonic
> time here,
> maybe we can replace the whole function as:
>
> return jiffies % HZ;
>
> But we need some tricks here to make sure the return value is correct,
> if HZ macro is greater than 1000 in some platform.
On most architectures, HZ is between 100 and 1000, I believe Alpha is the
only exception that goes as high as 1200. However, there is also a
jiffies_to_msecs() function that might do the exact right thing and
is normally very efficient.
The main downsides are that it loses precision on architectures with HZ=100,
and that it will jump once every 41 days on architectures with HZ=1000 when
jiffies overflows, as ULONG_MAX is not a multiple of 1000.
Do you know the specific requirement on the USB frame numbers? I don't know
enough about USB to tell if either of those matter.
Arnd
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