pwm: Add hardware readout infrastructure

Add a ->get_state() function to the pwm_ops struct to let PWM drivers
initialize the PWM state attached to a PWM device.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

2 files changed, 31 insertions, 0 deletions

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index c240b5437145..a909c64ee863 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -270,6 +270,9 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
                 pwm->hwpwm = i;
                 pwm->state.polarity = polarity;
 
+                if (chip->ops->get_state)
+                        chip->ops->get_state(chip, pwm, &pwm->state);
+
                 radix_tree_insert(&pwm_tree, pwm->pwm, pwm);
         }
 
diff --git a/include/linux/pwm.h b/include/linux/pwm.h
index 150563a806d6..33f8decd9f38 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -206,6 +206,29 @@ static inline void pwm_get_args(const struct pwm_device *pwm,
 
 static inline void pwm_apply_args(struct pwm_device *pwm)
 {
+        /*
+         * PWM users calling pwm_apply_args() expect to have a fresh config
+         * where the polarity and period are set according to pwm_args info.
+         * The problem is, polarity can only be changed when the PWM is
+         * disabled.
+         *
+         * PWM drivers supporting hardware readout may declare the PWM device
+         * as enabled, and prevent polarity setting, which changes from the
+         * existing behavior, where all PWM devices are declared as disabled
+         * at startup (even if they are actually enabled), thus authorizing
+         * polarity setting.
+         *
+         * Instead of setting ->enabled to false, we call pwm_disable()
+         * before pwm_set_polarity() to ensure that everything is configured
+         * as expected, and the PWM is really disabled when the user request
+         * it.
+         *
+         * Note that PWM users requiring a smooth handover between the
+         * bootloader and the kernel (like critical regulators controlled by
+         * PWM devices) will have to switch to the atomic API and avoid calling
+         * pwm_apply_args().
+         */
+        pwm_disable(pwm);
         pwm_set_polarity(pwm, pwm->args.polarity);
 }
 
@@ -217,6 +240,9 @@ static inline void pwm_apply_args(struct pwm_device *pwm)
  * @set_polarity: configure the polarity of this PWM
  * @enable: enable PWM output toggling
  * @disable: disable PWM output toggling
+ * @get_state: get the current PWM state. This function is only
+ *             called once per PWM device when the PWM chip is
+ *             registered.
  * @dbg_show: optional routine to show contents in debugfs
  * @owner: helps prevent removal of modules exporting active PWMs
  */
@@ -229,6 +255,8 @@ struct pwm_ops {
                             enum pwm_polarity polarity);
         int (*enable)(struct pwm_chip *chip, struct pwm_device *pwm);
         void (*disable)(struct pwm_chip *chip, struct pwm_device *pwm);
+        void (*get_state)(struct pwm_chip *chip, struct pwm_device *pwm,
+                          struct pwm_state *state);
 #ifdef CONFIG_DEBUG_FS
         void (*dbg_show)(struct pwm_chip *chip, struct seq_file *s);
 #endif