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 <jernej.skra...@siol.net> > > 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
