On Wed, Apr 07, 2021 at 07:46:58AM +0200, Uwe Kleine-König wrote: > On Tue, Apr 06, 2021 at 06:41:37PM +0200, Clemens Gruber wrote: > > If the flag PWM_STAGGERING_ALLOWED is set on a channel, the PWM driver > > may (if supported by the HW) delay the ON time of the channel relative > > to the channel number. > > This does not alter the duty cycle ratio and is only relevant for PWM > > chips with less prescalers than channels, which would otherwise assert > > multiple or even all enabled channels at the same time. > > > > If this feature is supported by the driver and the flag is set on > > multiple channels, their ON times are spread out to improve EMI and > > reduce current spikes. > > As said in reply to patch 4/8 already: I don't like this idea and > think this should be made explicit using a new offset member in struct > pwm_state instead. That's because I think that the wave form a PWM > generates should be (completely) defined by the consumer and not by a > mix between consumer and device tree. Also the consumer has no (sane) > way to determine if staggering is in use or not.
I don't think offsets are ideal for this feature: It makes it more cumbersome for the user, because he has to allocate the offsets himself instead of a simple on/off switch. The envisioned usecase is: "I want better EMI behavior and don't care about the individual channels no longer being asserted at the exact same time". > One side effect (at least for the pca9685) is that when programming a > new duty cycle it takes a bit longer than without staggering until the > new setting is active. Yes, but it can be turned off if this is a problem, now even per-PWM. > Another objection I have is that we already have some technical debt > because there are already two different types of drivers (.apply vs > .config+.set_polarity+.enable+.disable) and I would like to unify this > first before introducing new stuff. But there is already PWM_POLARITY_INVERTED, which can be set in the DT. I am only adding another flag. Thierry: What's your take on this? Thanks, Clemens
