Hello Uwe,
Thank you for taking the time to explain your thinking. On Mon, Dec 07, 2020 at 09:16:28AM +0100, Uwe Kleine-König wrote: > On Sun, Dec 06, 2020 at 02:19:41PM +0000, Sean Young wrote: > > On Sat, Dec 05, 2020 at 08:25:10PM +0100, Uwe Kleine-König wrote: > > > On Sat, Dec 05, 2020 at 05:34:44PM +0000, Sean Young wrote: > > > > What real life uses-cases are there for round down? If you want to round > > > > down, is there any need for round up? > > > > > > The scenario I have in mind is for driving a motor. I have to admit > > > however that usually the period doesn't matter much and it's the > > > duty_cycle that defines the motor's speed. So for this case the > > > conservative behaviour is round-down to not make the motor run faster > > > than expected. > > > > I am reading here that for driving motors, only the duty cycle matters, > > not the period. > > There is an upper limit (usually around 1 ms) for the period, but if you > choose 0.1 ms or 0.001 ms doesn't matter much AFAICT. > > @Thierry: Do you have further use cases in mind? > > > > For other usecases (fan, backlight, LED) exactness typically doesn't > > > matter that much. > > > > So, the use-cases you have are driving motor, fan, backlight, and led. > > And in all these cases the exact Hz does not matter. > > > > The only uses case where the exact Hz does matter is pwm-ir-tx. > > > > So, I gather there are no use-cases for round-down. Yes, should round-down > > be needed, then this is more difficult to implement if the driver always > > does a round-closest. But, since there is no reason to have round-down, > > this is all academic. > > > > Your policy of forcing new pwm drivers to use round-down is breaking > > pwm-ir-tx. > > So you're indeed suggesting that the "right" rounding strategy for > lowlevel drivers should be: > > - Use the period length closest to the requested period (in doubt round > down?) > - With the chosen period length use the biggest duty_cycle not bigger > than the requested duty_cycle. > > While this seems technically fine I think for maintenance this is a > nightmare. > > My preference would be to stick to the rounding strategy we used so far > (i.e.: > > - Use the biggest period length not bigger than the requested period > - With the chosen period length use the biggest duty_cycle not bigger > than the requested duty_cycle. > > ) and for pwm-ir-tx add support to the PWM API to still make it possible > (and easy) to select the best setting. > > The reasons why I think that this rounding-down strategy is the best > are (in order of importance): > > - It is easier to implement correctly [1] Yes, you are right. You have given a great example where a simple DIV_ROUND_CLOSEST() does not give the result you want. > - Same rounding method for period and duty cycle > - most drivers already do this (I think) > > The (IMHO nice) result would then mean: > > - All consumers can get the setting they want; and Once there is a nice pwm api for selecting round-nearest, then yes. For the uses cases you've given, fan, backlight, and led a round-nearest is the rounding they would want, I would expect. > - Code in lowlevel drivers is simple and the complexity is in common > code and so a single place. > > And it would also allow the pwm-ir-tx driver to notice if the PWM to be > used can for example only support frequencies under 400 kHz. I doubt pwm-ir-tx cares about this, however it is a nice-to-have. It would also be nice if the rounding could be used with atomic configuration as well. Please let me know when/if this new API exists for pwm so that pwm-ir-tx can select the right rounding. > [1] Consider a PWM with a parent frequency of 66 MHz, to select the > period you can pick an integer divider "div" resulting in the period > 4096 / (pclk * d). So the obvious implementation for round-nearest > would be: > > pclk = clk_get_rate(myclk); > div = DIV_ROUND_CLOSEST(NSEC_PER_SEC * 4096, targetperiod * pclk); Note NSEC_PER_SEC * 4096 >> 2^32 so this would need to be DIV_ROUND_CLOSEST_ULL. > , right? > > With targetperiod = 2641 ns this picks div = 23 and so a period of > 2698.2872200263505 ns (delta = 57.2872200263505 ns). > The optimal divider however is div = 24. (implemented period = > 2585.8585858585857 ns, delta = 55.14141414141448 ns) > > For round-down the correct implementation is: > > pclk = clk_get_rate(myclk); > div = DIV_ROUND_UP(NSEC_PER_SEC * 4096, targetperiod * pclk); > > Exercise for the reader: Come up with a correct implementation for > "round-nearest" and compare its complexity to the round-down code. To be fair, I haven't been been able to come up with a solution without control flow. Thank you for an interesting conversation about this. Sean