[v2] pwm: imx27: fix race condition .apply,.get_state

From: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>

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.

Note that this change merely implements a sensible heuristic.
The i.MX has a 4 slot FIFO to configure the duty cycle. This FIFO
cannot be read back in its entirety. The "write x then read back
x from hw" semantics are therefore not easily applicable.
With this change, the .get_state() function tries to wait for some
stabilization in the FIFO (empty state). In this state it keeps
applying the last value written to the sample register.

Signed-off-by: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>
---
 drivers/pwm/pwm-imx27.c | 50 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 3 deletions(-)

On Thu, Jan 26, 2023 at 04:03:13PM +0100, Leif Middelschulte wrote:
> From: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>
> 
> 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.
> 
> Note that this change merely implements a sensible heuristic.
> The i.MX has a 4 slot FIFO to configure the duty cycle. This FIFO
> cannot be read back in its entirety. The "write x then read back
> x from hw" semantics are therefore not easily applicable.
> With this change, the .get_state() function tries to wait for some
> stabilization in the FIFO (empty state). In this state it keeps
> applying the last value written to the sample register.
> 
> Signed-off-by: Leif Middelschulte <Leif.Middelschulte@klsmartin.com>
> ---
>  drivers/pwm/pwm-imx27.c | 50 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
> index 29a3089c534c..32389ca2da3e 100644
> --- a/drivers/pwm/pwm-imx27.c
> +++ b/drivers/pwm/pwm-imx27.c
> @@ -75,6 +75,7 @@
>  						   (x)) + 1)
>  
>  #define MX3_PWM_SWR_LOOP		5
> +#define MX3_PWM_FIFOAV_EMPTY_LOOP	4
>  
>  /* PWMPR register value of 0xffff has the same effect as 0xfffe */
>  #define MX3_PWMPR_MAX			0xfffe
> @@ -118,8 +119,28 @@ static void pwm_imx27_clk_disable_unprepare(struct pwm_imx27_chip *imx)
>  	clk_disable_unprepare(imx->clk_ipg);
>  }
>  
> +static int pwm_imx27_wait_fifo_empty(struct pwm_chip *chip,
> +				     struct pwm_device *pwm)
> +{
> +	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
> +	struct device *dev = chip->dev;
> +	unsigned int period_ms = DIV_ROUND_UP_ULL(pwm_get_period(pwm), NSEC_PER_MSEC);

Please don't use pwm_get_period. This one is intended for PWM consumers
only. Directly using pwm->state.period is fine however.

> +	int tries = MX3_PWM_FIFOAV_EMPTY_LOOP;
> +	int fifoav;
> +	u32 sr;
> +
> +	while (tries--) {
> +		sr = readl(imx->mmio_base + MX3_PWMSR);
> +		fifoav = FIELD_GET(MX3_PWMSR_FIFOAV, sr);
> +		if (fifoav == MX3_PWMSR_FIFOAV_EMPTY)
> +			return;
> +		msleep(period_ms);

This is a rather long sleep. Maybe check in each iteration that fifoav
decreases as expected?

> +	}
> +	dev_warn(dev, "FIFO has been refilled concurrently\n");
> +}
> +
>  static int pwm_imx27_get_state(struct pwm_chip *chip,
> -			       struct pwm_device *pwm, struct pwm_state *state)
> +				struct pwm_device *pwm, struct pwm_state *state)
>  {
>  	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
>  	u32 period, prescaler, pwm_clk, val;
> @@ -161,10 +182,33 @@ static int pwm_imx27_get_state(struct pwm_chip *chip,
>  	 * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
>  	 * use the cached value.
>  	 */
> -	if (state->enabled)
> +	if (state->enabled) {
> +		/*
> +		 * From the i.MX PWM reference manual:
> +		 * "A read on the sample register yields the current FIFO value that
> +		 *  is being used, or will be used, by the PWM for generation on the
> +		 *  output signal. Therefore, a write and a subsequent read on the
> +		 *  sample register may result in different values being obtained."
> +		 * Furthermore:
> +		 * "When a new value is written, the duty cycle changes after the
> +		 *  current period is over."
> +		 * Note "changes" vs. "changes to the given value"!
> +		 * Finally:
> +		 * "The PWM will run at the last set duty-cycle setting if all the
> +		 *  values of the FIFO has been utilized, until the FIFO is reloaded
> +		 *  or the PWM is disabled."
> +		 * Try to be at least a bit more deterministic about which value is
> +		 * read by waiting until the FIFO is empty. In this state the last/most
> +		 * recently pushed sample (duty cycle) value is continuously applied.
> +		 * Beware that this approach is still racy, as a new value could have
> +		 * been supplied and a period expired between the call of the wait
> +		 * function and the subsequent readl.
> +		 */
> +		pwm_imx27_wait_fifo_empty(chip, pwm);

Instead of issuing a warning, I'd return an error code if
wait_fifo_empty fails.

>  		val = readl(imx->mmio_base + MX3_PWMSAR);
> -	else
> +	} else {
>  		val = imx->duty_cycle;
> +	}
>  
>  	tmp = NSEC_PER_SEC * (u64)(val) * prescaler;
>  	state->duty_cycle = DIV_ROUND_UP_ULL(tmp, pwm_clk);

Best regards
Uwe