Hello Lokesh, On Tue, Feb 25, 2020 at 01:29:57PM +0530, Lokesh Vutla wrote: > On 25/02/20 12:18 PM, Uwe Kleine-König wrote: > > On Tue, Feb 25, 2020 at 10:32:42AM +0530, Lokesh Vutla wrote: > >> On 24/02/20 2:25 PM, Uwe Kleine-König wrote: > >>> On Mon, Feb 24, 2020 at 10:51:34AM +0530, Lokesh Vutla wrote: > >>>> omap->pdata->set_load(omap->dm_timer, true, load_value); > >>>> omap->pdata->set_match(omap->dm_timer, true, match_value); > >>> > >>> (Without having looked into the depths of the driver I assume > >>> .set_load() sets the period of the PWM and .set_match() the duty cycle.) > >> > >> Right. > >> > >>> > >>> What happens on a running PWM if you change the period? Consider you > >>> change from duty_cycle = 1000, period = 5000 to duty_cycle = 4000, > >>> period = 10000. As you set the period first, can it happen the hardware > >>> produces a cycle with duty_cycle = 1000, period = 10000? > >> > >> No. So, the current cycle is un affected with duty_cycle = 1000 and period = > >> 5000. Starting from next cycle new settings gets reflected with duty_cycle = > >> 4000 and period = 10000. > > > > Is the reference manual for this hardware publically available? > > AM335x TRM [0] Section 20.1.3.5 Pulse-Width Modulation (Page 4445). > > [0] http://www.ti.com/lit/ug/spruh73q/spruh73q.pdf Great. This is BTW an opportunity to increase your patch count: Create a patch that adds a reference to this document at the top of the driver. > > So the .set_load callback just writes a shadow register and .set_match > > latches it into hardware atomically with its own register changes? A > > comment in the source code about this would be good. Also if .set_load > > doesn't work without .set_match I wonder if it is sane to put their > > logic in two different functions. > > Just to give a little bit of background: Thanks, very appreciated. > - The omap timer is an upward counter that can be started and stopped at any time. > - Once the timer counter overflows, it gets loaded with a predefined load > value.(Or can be configured to not re load at all). > - Timer has a configurable output pin which can be toggled in the following two > cases: > - When the counter overflows > - When the counter matches with a predefined register(match register). > > Using this o/p pin the driver tries to generate a PWM with period = (OVERFLOW - > LOAD_VALUE) and duty_cycle = (MATCH_VALUE - LOAD_VALUE). > > .set_load will configure the load value .set_match will configure the match > value. The configured values gets effected only in the next cycle of PWM. Ah, so back to my original question: If you change from duty_cycle/period = 1000/5000 to duty_cycle/period = 4000/10000 and after you set the period but before you set the duty_cycle a period happens to end, you get indeed a cycle with mixed settings, right? Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ |
