Messages in this thread Patch in this message | | | From | Leif Middelschulte <> | Subject | [PATCH v2] pwm: imx27: fix race condition .apply,.get_state | Date | Thu, 26 Jan 2023 16:03:13 +0100 |
| |
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); + 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); + } + 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); 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); -- 2.39.1
| |