On Tue, Apr 06, 2021 at 06:41:39PM +0200, Clemens Gruber wrote:
> Previously, the last used PWM channel could change the global prescale
> setting, even if other channels are already in use.
>
> Fix it by only allowing the first enabled PWM 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.
>
> GPIOs do not count as enabled PWMs as they are not using the prescaler
> and can't change it.
>
> Signed-off-by: Clemens Gruber <clemens.gru...@pqgruber.com>
> ---
> Changes since v6:
> - Only allow the first PWM that is enabled to change the prescaler, not
> the first one that uses the prescaler
>
> drivers/pwm/pwm-pca9685.c | 66 +++++++++++++++++++++++++++++++++------
> 1 file changed, 56 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 24221ee7a77a..cf0c98e4ef44 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 is enabled 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.
> + * GPIOs do not count as enabled PWMs as they are not using the prescaler.
> */
>
> #define PCA9685_MODE1 0x00
> @@ -78,8 +78,9 @@
> struct pca9685 {
> struct pwm_chip chip;
> struct regmap *regmap;
> -#if IS_ENABLED(CONFIG_GPIOLIB)
> struct mutex lock;
> + DECLARE_BITMAP(pwms_enabled, PCA9685_MAXCHAN + 1);
> +#if IS_ENABLED(CONFIG_GPIOLIB)
> struct gpio_chip gpio;
> DECLARE_BITMAP(pwms_inuse, PCA9685_MAXCHAN + 1);
> #endif
> @@ -90,6 +91,22 @@ 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 lock mutex held */
> +static bool pca9685_prescaler_can_change(struct pca9685 *pca, int channel)
> +{
> + /* No PWM enabled: Change allowed */
> + if (bitmap_empty(pca->pwms_enabled, PCA9685_MAXCHAN + 1))
> + return true;
> + /* More than one PWM enabled: Change not allowed */
> + if (bitmap_weight(pca->pwms_enabled, PCA9685_MAXCHAN + 1) > 1)
> + return false;
> + /*
> + * Only one PWM enabled: Change allowed if the PWM about to
> + * be changed is the one that is already enabled
> + */
> + return test_bit(channel, pca->pwms_enabled);
Maybe this is a bit more effective?:
DECLARE_BITMAP(blablub, PCA9685_MAXCHAN + 1);
bitmap_zero(blablub, PCA9685_MAXCHAN + 1);
bitmap_set(blablub, channel);
return bitmap_subset(pca->pwms_enabled, blablub);
(but that's a minor issue, the suggested algorithm is correct.)
So:
Acked-by: Uwe Kleine-König <u.kleine-koe...@pengutronix.de>
(side-note: I wonder if the handling of the set-all channel is correct
here. But given that it is messy anyhow, (e.g. because setting some
state to this set-all channel doesn't influence pwm_get_state for the
individual channels) I don't object if there is another problem in this
corner case. IMHO just dropping this virtual channel would be nice.)
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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