Hi Clemens, see below.
On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
<clemens.gru...@pqgruber.com> wrote:
>
> Previously, the last used PWM channel could change the global prescale
> setting, even if other channels were already in use.
>
> Fix it by only allowing the first user of the prescaler to change the
> global chip-wide prescale setting. If there is more than one channel in
> use, the prescale settings resulting from the chosen periods must match.
>
> PWMs that are disabled or have a duty cycle of 0% or 100% are not
> considered to be using the prescaler as they have the full OFF or full
> ON bits set. This also applies to channels used as GPIOs.
>
> Signed-off-by: Clemens Gruber <clemens.gru...@pqgruber.com>
> ---
> drivers/pwm/pwm-pca9685.c | 51 +++++++++++++++++++++++++++++++++------
> 1 file changed, 44 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index ff916980de49..438492d4aed4 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -23,11 +23,11 @@
> #include <linux/bitmap.h>
>
> /*
> - * Because the PCA9685 has only one prescaler per chip, changing the period
> of
> - * one channel affects the period of all 16 PWM outputs!
> - * However, the ratio between each configured duty cycle and the chip-wide
> - * period remains constant, because the OFF time is set in proportion to the
> - * counter range.
> + * Because the PCA9685 has only one prescaler per chip, only the first
> channel
> + * that uses the prescaler is allowed to change the prescale register.
> + * PWM channels requested afterwards must use a period that results in the
> same
> + * prescale setting as the one set by the first requested channel, unless
> they
> + * use duty cycles of 0% or 100% (prescaler not used for full OFF/ON).
> */
>
> #define PCA9685_MODE1 0x00
> @@ -80,6 +80,8 @@ struct pca9685 {
> struct pwm_chip chip;
> struct regmap *regmap;
> bool staggered_outputs;
> + struct mutex prescaler_users_lock;
Keep things simple by re-using the "struct mutex lock" below?
This code isn't performance-intensive, so having a single lock for
pwm/gpio requests + pwm_apply() is probably ok.
> + DECLARE_BITMAP(prescaler_users, PCA9685_MAXCHAN + 1);
Rename to pwms_use_prescale ?
> #if IS_ENABLED(CONFIG_GPIOLIB)
> struct mutex lock;
> struct gpio_chip gpio;
> @@ -92,6 +94,18 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
> return container_of(chip, struct pca9685, chip);
> }
>
> +/* This function is supposed to be called with the prescaler_users_lock held
> */
> +static inline bool pca9685_may_change_prescaler(struct pca9685 *pca, int
> channel)
Drop the inline? Only the compiler knows if inlining this function makes sense
on a platform (armv7, x86, etc). Compilers are usually better at this then
humans...
Rename to pca9685_prescaler_can_change() ?
> +{
> + /*
> + * A PWM channel may only change the prescaler if there are no users
> of
> + * the prescaler yet or that same channel is the only one in use.
> + */
> + return bitmap_empty(pca->prescaler_users, PCA9685_MAXCHAN + 1) ||
> + (bitmap_weight(pca->prescaler_users, PCA9685_MAXCHAN + 1) ==
> 1 &&
> + test_bit(channel, pca->prescaler_users));
> +}
I found this logic expression quite complex to read. Perhaps simplify by using
a few steps? For example:
/* if prescaler not in use, we can always change it */
if (empty) return true;
/* if more than one pwm is using the prescaler, we can never change it */
if (weight > 1) return false;
/* one pwm is using the prescaler, we can only change it if it's us */
return test_bit(us);
> +
> static void pca9685_pwm_set_duty(struct pca9685 *pca, int channel, unsigned
> int duty)
> {
> unsigned int on, off;
> @@ -337,16 +351,25 @@ static int pca9685_pwm_apply(struct pwm_chip *chip,
> struct pwm_device *pwm,
> duty = PCA9685_COUNTER_RANGE * state->duty_cycle;
> duty = DIV_ROUND_CLOSEST_ULL(duty, state->period);
>
> + mutex_lock(&pca->prescaler_users_lock);
> +
> if (!state->enabled || duty < 1) {
> pca9685_pwm_set_duty(pca, pwm->hwpwm, 0);
> - return 0;
> + goto prescaler_unused;
> } else if (duty == PCA9685_COUNTER_RANGE) {
> pca9685_pwm_set_duty(pca, pwm->hwpwm, duty);
> - return 0;
> + goto prescaler_unused;
> }
>
> regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
> if (prescale != val) {
> + if (!pca9685_may_change_prescaler(pca, pwm->hwpwm)) {
> + mutex_unlock(&pca->prescaler_users_lock);
> + dev_err(chip->dev,
> + "prescaler not set: already in use with
> different setting!\n");
> + return -EBUSY;
> + }
> +
> /*
> * Putting the chip briefly into SLEEP mode
> * at this point won't interfere with the
> @@ -364,6 +387,14 @@ static int pca9685_pwm_apply(struct pwm_chip *chip,
> struct pwm_device *pwm,
> }
>
> pca9685_pwm_set_duty(pca, pwm->hwpwm, duty);
> +
> + set_bit(pwm->hwpwm, pca->prescaler_users);
> + mutex_unlock(&pca->prescaler_users_lock);
> + return 0;
> +
> +prescaler_unused:
> + clear_bit(pwm->hwpwm, pca->prescaler_users);
> + mutex_unlock(&pca->prescaler_users_lock);
> return 0;
> }
The need for the mutex makes this function quite "messy": we have to guard all
the exits, and that's easy to forget.
Maybe simplify the function by moving the mutex to a helper?
Example:
static int __pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
const struct pwm_state *state)
{
... just do stuff and don't worry about the mutex
}
static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
const struct pwm_state *state)
{
/* document why we serialize pwm_apply */
mutex_lock();
__pca9685_pwm_apply(chip, pwm, state);
mutex_unlock();
}
>
> @@ -422,7 +453,11 @@ static void pca9685_pwm_free(struct pwm_chip *chip,
> struct pwm_device *pwm)
> {
> struct pca9685 *pca = to_pca(chip);
>
> + mutex_lock(&pca->prescaler_users_lock);
> + clear_bit(pwm->hwpwm, pca->prescaler_users);
> pca9685_pwm_set_duty(pca, pwm->hwpwm, 0);
> + mutex_unlock(&pca->prescaler_users_lock);
> +
> pm_runtime_put(chip->dev);
> pca9685_pwm_clear_inuse(pca, pwm->hwpwm);
> }
> @@ -463,6 +498,8 @@ static int pca9685_pwm_probe(struct i2c_client *client,
>
> i2c_set_clientdata(client, pca);
>
> + mutex_init(&pca->prescaler_users_lock);
> +
> regmap_read(pca->regmap, PCA9685_MODE2, ®);
>
> if (device_property_read_bool(&client->dev, "invert"))
> --
> 2.29.2
>
