On Thu, Mar 20, 2014 at 8:57 AM, Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > On Tue, Mar 18, 2014 at 04:47:36PM -0700, Tim Kryger wrote: > Perhaps the comment for enum pwm_polarity in include/linux/pwm.h makes > this more obvious. What you do here is invert the duty cycle, rather > than the polarity. While it is true that the result is the same for > things like LEDs or backlight (because the signal power remains the > same), but there's a slight difference to what the PWM signal looks > like. Thanks, I missed that comment before. >> Does polarity influence the output while a PWM is disabled? > > Yes, there is apparently hardware where the polarity causes the PWM pin > to be 1 when the PWM is disabled. But that's really a separate issue. Do you have a preference on how this should be handled? > Things are starting to get confusing here. Looking at the register > definitions, there's PWM_CONTROL_ENABLE_SHIFT(chan), which I suspect > controls whether or not a channel is enabled (if that's not the case > then please add a comment that explains what it does). I've tried to do this but the unfortunate name for these bits and their nuanced behavior makes it difficult. > > But a little further up you said that the hardware does only support a > configure operation and not an enable/disable. If you define disabled as zero duty output, then this is true. When the smooth bit is off and the "enable" bit is off output is 100% duty. > > The comment above further confuses me. What I read from it is that you > can in fact disable a channel by clearing the "enable" bit in the > control register. But the reason why you don't do it that way is because > that change won't take effect until "settings start to take effect". So > in order to disable the PWM immediately you resort to writing a 0 duty > cycle. Sorry if my comments were confusing. New settings are only applied on a rising edge of the "enable" bit. You should think of it more as a trigger bit than an enable. > > Perhaps I misunderstood, in which case it might be good to revise that > comment to be more explicit or accurate. Perhaps it would be clearest to deviate from the hw docs and rename PWM_CONTROL_ENABLE_SHIFT to PWM_CONTROL_TRIGGER_SHIFT to more closely match its function. What do you think of the following? /* * New duty and period settings are only reflected in the PWM output * after a rising edge of the trigger bit. After a rising edge, if the * smooth bit is set, the settings are also delayed until the current * period has completed. Furthermore, if the smooth bit is set, the PWM * continues to output a waveform based on the old settings during the * time that the trigger bit is low. Otherwise the output is a constant * high signal while the trigger bit is low. */ -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
