Hi,
Dne četrtek, 18. januar 2018 ob 11:58:41 CET je Maxime Ripard napisal(a):
> Hi,
>
> On Wed, Jan 17, 2018 at 09:14:11PM +0100, Jernej Skrabec wrote:
> > This commit changes formula from this:
> >
> > Freq = (parent_freq * N * K) / (M * P)
> >
> > to this:
> >
> > Freq = (parent_freq / M) * N * K / P
> >
> > This improves situation when N is in the range 1-255. PLL parent clock
> > is almost always 24 MHz, which means that for N >= 180 original formula
> > overflows and result becomes useless. Situation can be improved if M is
> > used as predivider as it can be seen in the second formula. That way at
> > least M > 1 is considered, but it still leaves small gap for wrong result
> > when M = 1 and N >= 180.
> >
> > Using M as predivider shouldn't cause any issue, because it is in range
> > 1-4 at most, so there is no or only minimal rounding error.
> >
> > Signed-off-by: Jernej Skrabec
>
> I'd really prefer to stick to the formula documented and that we've
> used so far. NKMP clocks are most notably used for the CPU PLLs and
> I've debugged way too many cpufreq bugs already :)
>
> What about using long long types for the parent * n * k result?
Yes, using long long is the best possible solution and covers all cases
whereas this patch does not.
I thought that do_div() would cause a lot of overhead, but I noticed that it's
not big if both numbers fit in 32 bit, which in our case is true most of the
time.
I will make a helper function for calculating rate, since using long long
needs more than one line of code.
Best regards,
Jernej
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