Hi Guenter, On Thu, Oct 21, 2010 at 07:43:46AM -0700, Guenter Roeck wrote: > > > > > > The combination of DIV_ROUND_UP() and DIV_ROUND_CLOSEST() causes inconsistency. > > > > > > Assume num_speed = 8, pwm is set to 128. > > > > > > set: 128 * (8 - 1) / 255 = 3.513 ==> 4 > > > get: 4 * 255 / (8 - 1) = 145.7 ==> 146 > > > set: 146 * (8 - 1) / 255 = 4.007 ==> 5 > > > get: 5 * 255 / (8 - 1) = 182.142 ==> 182 > > > set: 182 * (8 - 1) / 255 = 4.996 ==> 5 > > > > > > Unless there is a really good reason to use DIV_ROUND_UP(), you might > > > want to use DIV_ROUND_CLOSEST() instead. > > > > This choice is coherent with the rpm interface one and the reason is the > > same: start the fan even with a low value. In your example, 36 is first > > speed threshold. > > > Yes, but here it causes an inconsistency between setting and reporting. > I don't expect the speed to change if I set the same value that was read. > Exactly this happens if one writes 146 in my example. That is much worse > than a potential startup problem, or the observation that pwm values below X > don't start the fan. Mmm. Convert a speed index into a low round pwm value (and not use DIV_ROUND_CLOSEST() at all) fix the inconsistency too. If you agree, I would prefer this option. Thanks, Simon
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