Hi Sven, On Fri, Jan 29, 2021 at 08:42:13AM -0500, Sven Van Asbroeck wrote: > On Mon, Jan 11, 2021 at 3:35 PM Uwe Kleine-König > <u.kleine-koe...@pengutronix.de> wrote: > > > > My position here is: A consumer should disable a PWM before calling > > pwm_put. The driver should however not enforce this and so should not > > modify the hardware state in .free(). > > > > Also .probe should not change the PWM configuration. > > I agree that this is the most user-friendly behaviour. > > The problem however with the pca9685 is that it has many degrees of > freedom: there are many possible register values which produce the same > physical chip outputs. > > This could lead to a situation where, if .probe() does not reset the register > values, subsequent writes may lead to different outputs than expected. > > One possible solution is to write .get_state() so that it always reads the > correct state, even if "unconventional" register settings are present, i.e. > those written by an outside entity, e.g. a bootloader. Then write that > state back using driver conventions. > > This may be trickier than it sounds - after all we've learnt that the pca9685 > looks simple on the surface, but is actually quite challenging to get right. > > Clemens, Uwe, what do you think?
Ok, so you suggest we extend our get_state logic to deal with cases like the following: If neither full OFF nor full ON is set && ON == OFF, we should probably set the full OFF bit to disable the PWM and log a warning message? (e.g. "invalid register setting detected: pwm disabled" ?) If the ON registers are set and the nxp,staggered-outputs property is not, I'd calculate (off - on) & 4095, set the OFF register to that value and clear the ON register. And then call our get_state in .probe, followed by a write of the resulting / fixed-up state? This would definitely solve the problem of invalid/unconventional values set by the bootloader and avoid inconsistencies. Sounds good to me! If Thierry and Uwe have no objections, I can send out a new round of patches in the upcoming weeks. My current goal is to get the changes into 5.13. Thanks, Clemens
