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Message-ID: <20190604091425.GL2781@lahna.fi.intel.com>
Date: Tue, 4 Jun 2019 12:14:25 +0300
From: Mika Westerberg <mika.westerberg@...ux.intel.com>
To: Sven Van Asbroeck <thesven73@...il.com>
Cc: Thierry Reding <thierry.reding@...il.com>,
linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH] pwm: pca9685: fix pwm/gpio inter-operation
On Mon, Jun 03, 2019 at 11:12:23AM -0400, Sven Van Asbroeck wrote:
> This driver allows pwms to be requested as gpios via gpiolib.
> Obviously, it should not be allowed to request a gpio when its
> corresponding pwm is already requested (and vice versa).
> So it requires some exclusion code.
>
> Given that the pwm and gpio cores are not synchronized with
> respect to each other, this exclusion code will also require
> proper synchronization.
>
> Such a mechanism was in place, but was inadvertently removed
> by Uwe's clean-up patch.
>
> Upon revisiting the synchronization mechanism, we found that
> theoretically, it could allow two threads to successfully
> request conflicting pwms / gpios.
>
> Replace with a bitmap which tracks pwm in-use, plus a mutex.
> As long as pwm and gpio's respective request/free functions
> modify the in-use bitmap while holding the mutex, proper
> synchronization will be guaranteed.
>
> Reported-by: YueHaibing <yuehaibing@...wei.com>
> Fixes: e926b12c611c ("pwm: Clear chip_data in pwm_put()")
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Link: https://lkml.org/lkml/2019/5/31/963
> Signed-off-by: Sven Van Asbroeck <TheSven73@...il.com>
> ---
>
> This approach will also prevent the request of the "all" pwm channel, if any
> other pwm channel is already in use. Is this correct behaviour?
Sounds correct to me.
> drivers/pwm/pwm-pca9685.c | 64 +++++++++++++++++++++------------------
> 1 file changed, 35 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 567f5e2771c4..f9927cd106d0 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -31,6 +31,7 @@
> #include <linux/slab.h>
> #include <linux/delay.h>
> #include <linux/pm_runtime.h>
> +#include <linux/bitmap.h>
>
> /*
> * Because the PCA9685 has only one prescaler per chip, changing the period of
> @@ -85,6 +86,7 @@ struct pca9685 {
> #if IS_ENABLED(CONFIG_GPIOLIB)
> struct mutex lock;
> struct gpio_chip gpio;
> + DECLARE_BITMAP(pwms_inuse, PCA9685_MAXCHAN);
> #endif
> };
>
> @@ -97,48 +99,45 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
> static int pca9685_pwm_gpio_request(struct gpio_chip *gpio, unsigned int offset)
> {
> struct pca9685 *pca = gpiochip_get_data(gpio);
> - struct pwm_device *pwm;
>
> mutex_lock(&pca->lock);
>
> - pwm = &pca->chip.pwms[offset];
> -
> - if (pwm->flags & (PWMF_REQUESTED | PWMF_EXPORTED)) {
> + if (test_and_set_bit(offset, pca->pwms_inuse)) {
> mutex_unlock(&pca->lock);
> return -EBUSY;
> }
>
> - pwm_set_chip_data(pwm, (void *)1);
> -
> mutex_unlock(&pca->lock);
> pm_runtime_get_sync(pca->chip.dev);
> return 0;
> }
>
> -static bool pca9685_pwm_is_gpio(struct pca9685 *pca, struct pwm_device *pwm)
> +static bool
> +pca9685_pwm_test_set_inuse(struct pca9685 *pca, struct pwm_device *pwm)
Can we call it pca9685_pwm_test_and_set_inuse() following naming of
test_and_set_bit()?
> {
> - bool is_gpio = false;
> + bool is_inuse;
>
> mutex_lock(&pca->lock);
> + /*
> + * Check if any of the PWMs are requested and in that case
> + * prevent using the "all LEDs" channel.
> + */
> + if (pwm->hwpwm >= PCA9685_MAXCHAN &&
> + !bitmap_empty(pca->pwms_inuse, PCA9685_MAXCHAN))
> + is_inuse = true;
> + else
> + is_inuse = test_and_set_bit(pwm->hwpwm, pca->pwms_inuse);
> + mutex_unlock(&pca->lock);
>
> - if (pwm->hwpwm >= PCA9685_MAXCHAN) {
> - unsigned int i;
> -
> - /*
> - * Check if any of the GPIOs are requested and in that case
> - * prevent using the "all LEDs" channel.
> - */
> - for (i = 0; i < pca->gpio.ngpio; i++)
> - if (gpiochip_is_requested(&pca->gpio, i)) {
> - is_gpio = true;
> - break;
> - }
> - } else if (pwm_get_chip_data(pwm)) {
> - is_gpio = true;
> - }
> + return is_inuse;
> +}
>
> +static void pca9685_pwm_clear_inuse(struct pca9685 *pca, struct pwm_device *pwm)
I think it might be better if you provide __pca9685_pwm_clear_inuse()
that does not take the lock and then pca9685_pwm_clear_inuse() that just
calls the former. Then ->
> +{
> + mutex_lock(&pca->lock);
> + if (pwm->hwpwm < PCA9685_MAXCHAN)
> + clear_bit(pwm->hwpwm, pca->pwms_inuse);
> mutex_unlock(&pca->lock);
> - return is_gpio;
> }
>
> static int pca9685_pwm_gpio_get(struct gpio_chip *gpio, unsigned int offset)
> @@ -170,12 +169,11 @@ static void pca9685_pwm_gpio_set(struct gpio_chip *gpio, unsigned int offset,
> static void pca9685_pwm_gpio_free(struct gpio_chip *gpio, unsigned int offset)
> {
> struct pca9685 *pca = gpiochip_get_data(gpio);
> - struct pwm_device *pwm;
>
> + mutex_lock(&pca->lock);
> pca9685_pwm_gpio_set(gpio, offset, 0);
> pm_runtime_put(pca->chip.dev);
> - mutex_lock(&pca->lock);
> - pwm = &pca->chip.pwms[offset];
> + clear_bit(offset, pca->pwms_inuse);
-> you can call
__pca9685_pwm_clear_inuse()
It feels more "consistent" wrt setting the bit.
> mutex_unlock(&pca->lock);
> }
>
> @@ -228,12 +226,17 @@ static int pca9685_pwm_gpio_probe(struct pca9685 *pca)
> return devm_gpiochip_add_data(dev, &pca->gpio, pca);
> }
> #else
> -static inline bool pca9685_pwm_is_gpio(struct pca9685 *pca,
> +static inline bool pca9685_pwm_test_set_inuse(struct pca9685 *pca,
> struct pwm_device *pwm)
> {
> return false;
> }
>
> +static inline void
> +pca9685_pwm_clear_inuse(struct pca9685 *pca, struct pwm_device *pwm)
> +{
> +}
> +
> static inline int pca9685_pwm_gpio_probe(struct pca9685 *pca)
> {
> return 0;
> @@ -417,7 +420,7 @@ static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
> {
> struct pca9685 *pca = to_pca(chip);
>
> - if (pca9685_pwm_is_gpio(pca, pwm))
> + if (pca9685_pwm_test_set_inuse(pca, pwm))
> return -EBUSY;
> pm_runtime_get_sync(chip->dev);
>
> @@ -426,8 +429,11 @@ static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
>
> static void pca9685_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
> {
> + struct pca9685 *pca = to_pca(chip);
> +
> pca9685_pwm_disable(chip, pwm);
> pm_runtime_put(chip->dev);
> + pca9685_pwm_clear_inuse(pca, pwm);
> }
>
> static const struct pwm_ops pca9685_pwm_ops = {
> --
> 2.17.1
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