Re: [PATCH 1/2] pwm: atmel-tcb: Fix race condition and convert to guards

[Nicolas Ferre <nicolas.ferre@microchip.com>]

On 09/07/2024 at 12:18, Uwe Kleine-König wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> The hardware only supports a single period length for both PWM outputs. So
> atmel_tcb_pwm_config() checks the configuration of the other output if it's
> compatible with the currently requested setting. The register values are
> then actually updated in atmel_tcb_pwm_enable(). To make this race free
> the lock must be held during the whole process, so grab the lock in
> .apply() instead of individually in atmel_tcb_pwm_disable() and
> atmel_tcb_pwm_enable() which then also covers atmel_tcb_pwm_config().
> 
> To simplify handling, use the guard helper to let the compiler care for
> unlocking. Otherwise unlocking would be more difficult as there is more
> than one exit path in atmel_tcb_pwm_apply().
> 
> Fixes: 9421bade0765 ("pwm: atmel: add Timer Counter Block PWM driver")
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>

And I like the conversion to the "guard" lock helper.
Best regards,
   Nicolas

> ---
>   drivers/pwm/pwm-atmel-tcb.c | 12 +++++-------
>   1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-atmel-tcb.c b/drivers/pwm/pwm-atmel-tcb.c
> index 528e54c5999d..aca11493239a 100644
> --- a/drivers/pwm/pwm-atmel-tcb.c
> +++ b/drivers/pwm/pwm-atmel-tcb.c
> @@ -81,7 +81,8 @@ static int atmel_tcb_pwm_request(struct pwm_chip *chip,
>          tcbpwm->period = 0;
>          tcbpwm->div = 0;
> 
> -       spin_lock(&tcbpwmc->lock);
> +       guard(spinlock)(&tcbpwmc->lock);
> +
>          regmap_read(tcbpwmc->regmap, ATMEL_TC_REG(tcbpwmc->channel, CMR), &cmr);
>          /*
>           * Get init config from Timer Counter registers if
> @@ -107,7 +108,6 @@ static int atmel_tcb_pwm_request(struct pwm_chip *chip,
> 
>          cmr |= ATMEL_TC_WAVE | ATMEL_TC_WAVESEL_UP_AUTO | ATMEL_TC_EEVT_XC0;
>          regmap_write(tcbpwmc->regmap, ATMEL_TC_REG(tcbpwmc->channel, CMR), cmr);
> -       spin_unlock(&tcbpwmc->lock);
> 
>          return 0;
>   }
> @@ -137,7 +137,6 @@ static void atmel_tcb_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm,
>          if (tcbpwm->duty == 0)
>                  polarity = !polarity;
> 
> -       spin_lock(&tcbpwmc->lock);
>          regmap_read(tcbpwmc->regmap, ATMEL_TC_REG(tcbpwmc->channel, CMR), &cmr);
> 
>          /* flush old setting and set the new one */
> @@ -172,8 +171,6 @@ static void atmel_tcb_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm,
>                               ATMEL_TC_SWTRG);
>                  tcbpwmc->bkup.enabled = 0;
>          }
> -
> -       spin_unlock(&tcbpwmc->lock);
>   }
> 
>   static int atmel_tcb_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm,
> @@ -194,7 +191,6 @@ static int atmel_tcb_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm,
>          if (tcbpwm->duty == 0)
>                  polarity = !polarity;
> 
> -       spin_lock(&tcbpwmc->lock);
>          regmap_read(tcbpwmc->regmap, ATMEL_TC_REG(tcbpwmc->channel, CMR), &cmr);
> 
>          /* flush old setting and set the new one */
> @@ -256,7 +252,6 @@ static int atmel_tcb_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm,
>          regmap_write(tcbpwmc->regmap, ATMEL_TC_REG(tcbpwmc->channel, CCR),
>                       ATMEL_TC_SWTRG | ATMEL_TC_CLKEN);
>          tcbpwmc->bkup.enabled = 1;
> -       spin_unlock(&tcbpwmc->lock);
>          return 0;
>   }
> 
> @@ -341,9 +336,12 @@ static int atmel_tcb_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>   static int atmel_tcb_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>                                 const struct pwm_state *state)
>   {
> +       struct atmel_tcb_pwm_chip *tcbpwmc = to_tcb_chip(chip);
>          int duty_cycle, period;
>          int ret;
> 
> +       guard(spinlock)(&tcbpwmc->lock);
> +
>          if (!state->enabled) {
>                  atmel_tcb_pwm_disable(chip, pwm, state->polarity);
>                  return 0;
> 
> base-commit: 120a528213b6693214e3cbc24a9c3052a4b1024b
> --
> 2.43.0
>