Messages in this thread | | | Subject | Re: (EXT) Re: (EXT) Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element | From | Matthias Schiffer <> | Date | Tue, 07 Apr 2020 15:00:43 +0200 |
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On Mon, 2020-04-06 at 11:51 +0200, Thierry Reding wrote: > > On Sat, Apr 04, 2020 at 04:17:00PM -0400, Sven Van Asbroeck wrote: > > > > It does look like we have a square peg (this chip) in a round hole > > (the > > standard assumptions the pwm core makes) ? > > There are other chips where a single period is shared across multiple > PWM channels. Typically what we do there is once a period is > configured > for a given channel, all subsequent PWM channel configurations must > use > the same period, or otherwise the driver will return an error code. > > See for example: > > - stm32_pwm_config() in drivers/pwm/pwm-stm32.c > - lpc18xx_pwm_config() in drivers/pwm/pwm-lpc18xx-sct.c > - pwm_imx_tpm_apply_hw() in drivers/pwm/pwm-imx-tpm.c > - fsl_pwm_apply_config() in drivers/pwm/pwm-fsl-ftm.c > > The rationale behind that is that we must not change a PWM > configuration > without a consumer having explicitly requested it.
These implementations don't deal with the issue in a consistent way either though:
1) stm32_pwm_config() only checks for channels that are actually enabled, regardless if they're requested
2) lpc18xx_pwm_config() checks requested channels instead
"2)" seems more correct to me, as the parameters of a requested channel can't be changed by another user, but it seems to prevent certain sequences. I don't have a good grasp of the the usual PWM request control flow - is it correct that the PWM state is not updated with the PWM args from OF when the PWM is requested? I only see pwm_adjust_config() doing something like that, which is not used in many places (and nothing preventing races) - but I might be overlooking something.
Meaning, is it possible that two drivers request PWMs from the same pwmchip of type "2)" (or even a single driver using two PWMs) without configuring them right away may get into a situation where neither can set up a period at all even when all users want to use the same period?
Matthias
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