Messages in this thread | | | Date | Wed, 25 Jan 2023 17:43:16 +0100 | From | Uwe Kleine-König <> | Subject | Re: [PATCH] pwm: imx27: fix race condition .apply,.get_state |
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Hello Leif,
first of all thanks for the patch.
On Wed, Jan 25, 2023 at 05:01:42PM +0100, Leif Middelschulte wrote: > From: Leif Middelschulte <Leif.Middelschulte@klsmartin.com> > > A race condition might occur, ultimately leading to switching off the > PWM that is supposed to be turned on. > The condition is more likely, if `CONFIG_PWM_DEBUG` is set and the PWM > has been enabled before Linux is booted.
As I understand it there is no problem if PWM_DEBUG is off, isn't it?
> After writing some value to the register linked to the duty cycle > (`MX3_PWMSAR`), the related debug function > (`core.c:pwm_apply_state_debug`) reads back (`.get_state`) > a wrong value (`0`) as the configured duty cycle. This value is stored > as part of a temporary state variable that it subsequently reapplies > to the PWM for testing purposes. Which, effectively, turns off the PWM.
I thought the thing is: Reading PWMSAR yields the duty_cycle that is currently in use. Now if .apply() is called with a new value for PWMSAR and immediately after that .get_state() reads out PWMSAR the previous period (with the previous duty_cycle) probably isn't completed yet and so the old value is read.
In this case it wouldn't always be 0 which is read. (Hmm, but with the conversion we had about this issue, my theory sounds wrong?!)
Maybe instead of waiting in .apply() (which hurts active consumers), only wait in .get_state() until MX3_PWMSR_FIFOAV drops to zero?
Apart from that, the markdown(?) style you use is unusual for kernel commit logs and comments. I'd write:
With CONFIG_PWM_DEBUG=y after writing a value to the PWMSAR register in .apply(), the register is read in .get_state(). Unless a period completed in the meantime, this read yields the previously used duty cycle configuration. As the PWM_DEBUG code applies the read out configuration for testing purposes this effectively undoes the intended effect by rewriting the previous hardware state.
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ | [unhandled content-type:application/pgp-signature] | |