linux-kernel - Re: [PATCH] pwm: imx27: fix race condition .apply,.get

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Message-ID: <20230125164316.wkoi4qfzbhna3h6g@pengutronix.de>
Date:   Wed, 25 Jan 2023 17:43:16 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Leif Middelschulte <leif.middelschulte@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Leif Middelschulte <Leif.Middelschulte@...martin.com>,
        linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pwm: imx27: fix race condition .apply,.get_state

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@...martin.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/ |

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