Hi Uwe, On Mon, Dec 07, 2020 at 02:52:09PM +0100, Uwe Kleine-König wrote: > On Mon, Dec 07, 2020 at 09:43:20AM +0000, Sean Young wrote: > > 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? > > I asked in the hardware department of the company I work for and they > had another usecase: Motors where for example a 1 ms pulse means "move > forwards" and 2 ms means "move backwards". They had the same idea as I > had: You want to know beforehand the result of a given > pwm_apply_state().
That sounds good, that would be nice. > > > > > 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. > > maybe, yes. Maybe it is also not important enough to spend the extra > cycles getting round nearest and so sticking to round-down is good > enough. > > > > - 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. > > I cannot follow, you created 11fc4edc483bea8bf0efa0cc726886d2342f6fa6 > because 476.2 Mhz was too bad. So you seem to be interested in > deviations and part of the problem is that you don't get feedback about > how your request is fulfilled. Right, that's true. > > Please let me know when/if this new API exists for pwm so that pwm-ir-tx > > can select the right rounding. > > Given that the bcm2835 driver is quite trivial I would be happy to > create a series that "fixes" the driver to round down and provide a > prototype for pwm_round_nearest for you to test on pwm-ir-tx. A willing > tester and a real use-case were the single two things that stopped me > investing time here. pwm-ir-tx does not just use the bcm2845 driver/rpi. There is the Firefly ROC-RK3308-CC board which uses pwm-ir-tx with a different pwm dirver. Also all you need is a infrared led, and a resistor to stop the led from burning out, to create your own infrared emitter. So, users can easily add pwm-ir-tx to their systems. Having said that I'm happy to test the rpi. I would attach a logic analyser and check the period. Sean
