Hello Baolin, On Fri, Aug 16, 2019 at 10:44:41AM +0800, Baolin Wang wrote: > On Thu, 15 Aug 2019 at 20:25, Uwe Kleine-König > <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > On Thu, Aug 15, 2019 at 07:05:53PM +0800, Baolin Wang wrote: > > > On Thu, 15 Aug 2019 at 18:11, Uwe Kleine-König > > > <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > > > > > Hello, > > > > > > > > On Thu, Aug 15, 2019 at 05:34:02PM +0800, Baolin Wang wrote: > > > > > On Thu, 15 Aug 2019 at 16:54, Uwe Kleine-König > > > > > <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > > > On Thu, Aug 15, 2019 at 04:16:32PM +0800, Baolin Wang wrote: > > > > > > > On Thu, 15 Aug 2019 at 14:15, Uwe Kleine-König > > > > > > > <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > > > > > On Thu, Aug 15, 2019 at 11:34:27AM +0800, Baolin Wang wrote: > > > > > > > > > On Wed, 14 Aug 2019 at 23:03, Uwe Kleine-König > > > > > > > > > <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > > > > > > > > > On Wed, Aug 14, 2019 at 08:46:11PM +0800, Baolin Wang wrote: > > > > > > > > > > > + * To keep the maths simple we're always using MOD = SPRD_PWM_MOD_MAX. > > > > > > > > > > > > > > > > > > > > Did you spend some thoughts about how wrong your period can get because > > > > > > > > > > of that "lazyness"? > > > > > > > > > > > > > > > > > > > > Let's assume a clk rate of 100/3 MHz. Then the available period lengths > > > > > > > > > > are: > > > > > > > > > > > > > > > > > > > > PRESCALE = 0 -> period = 7.65 µs > > > > > > > > > > PRESCALE = 1 -> period = 15.30 µs > > > > > > > > > > ... > > > > > > > > > > PRESCALE = 17 -> period = 137.70 µs > > > > > > > > > > PRESCALE = 18 -> period = 145.35 µs > > > > > > > > > > > > > > > > > > > > So the error can be up to (nearly) 7.65 µs (or in general > > > > > > > > > > > > > > > > > > Yes, but for our use case (pwm backlight), the precision can meet our > > > > > > > > > requirement. Moreover, we usually do not change the period, just > > > > > > > > > adjust the duty to change the back light. > > > > > > > > > > > > > > > > Is this a license requirement for you SoC to only drive a backlight with > > > > > > > > the PWM? The idea of having a PWM driver on your platform is that it can > > > > > > > > also be used to control a step motor or a laser. > > > > > > > > > > > > > > Not a license requirement. Until now we have not got any higher > > > > > > > precision requirements, and we've run this driver for many years in > > > > > > > our downstream kernel. > > > > > > > > > > > > I understood that you're not ambitious to do something better than "it > > > > > > worked for years". > > > > > > > > > > How do you know that? > > > > > > > > I showed you how you could match the requested PWM output better and > > > > you refused telling it worked for years and the added precision isn't > > > > necessary for a backlight. > > > > > > Please I said the reason, it is not that I do not want a better > > > precision. The problem is we do not know how much precision to be > > > asked by users if no use case > > > > I don't understand the problem here. If you are asked for period = > > 145340 ns and configure the hardware to yield 137700 ns in reply to that > > but you could provide 144780 ns I don't understand why you need a use > > case as 144780 ns is objectively better than 137700 ns. A better match > > You are wrong, we will provide 145350 ns with > DIV_ROUND_CLOSEST_ULL()., which is better than your 144780. There are two problems with your statement: - You're ignoring the fact that I base my argumentation on not using DIV_ROUND_CLOSEST_ULL because it has downsides I pointed out to you. If my suggested algorithm targeted a closest match (and not closest-but-not-bigger) it would pick 145350 ns, too. (I didn't check if something in the interval [145331, 145349] could be achieved. If there is, this should of course be preferred.) I obviously would have to pick a different example request to show that "targeting nearest with MOD always 255" is for some requests worse (probably by a similar factor) than "targeting nearest". It's not surprising that your apple is a better apple than my orange. - It's not objective that when 145340 ns was requested 145350 ns is better than 144780 ns. Some approximations are obviously better than others, but these two are not comparable in a way that all PWM consumers agree. I'm not perfect and I do mistakes of course. But I'm still convinced that my argumentation here is right. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |
