[PATCH v3 3/3] input: gpio-keys: Use hrtimer for software debounce, if possible

[Paul Cercueil]

We want to be able to report the input event as soon as the debounce
delay elapsed. However, the current code does not really ensure that,
as it uses the jiffies-based schedule_delayed_work() API. With a small
enough HZ value (HZ <= 100), this results in some input events being
lost, when a key is quickly pressed then released (on a human's time
scale).

Switching to hrtimers fixes this issue, and will work even on extremely
low HZ values (tested at HZ=24). This is however only possible if
reading the GPIO is possible without sleeping. If this condition is not
met, the previous approach of using a jiffies-based timer is taken.

Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
---

Notes:
    v2: HRTIMER_MODE_REL_SOFT -> HRTIMER_MODE_REL
    
    v3: Only use a hrtimer-based timer if we know that reading the GPIO
        will never sleep.

 drivers/input/keyboard/gpio_keys.c | 69 ++++++++++++++++++++++++------
 1 file changed, 56 insertions(+), 13 deletions(-)

diff --git a/drivers/input/keyboard/gpio_keys.c b/drivers/input/keyboard/gpio_keys.c
index 4b92f49decef..046d9dffa171 100644
--- a/drivers/input/keyboard/gpio_keys.c
+++ b/drivers/input/keyboard/gpio_keys.c
@@ -41,6 +41,7 @@ struct gpio_button_data {
 	unsigned int release_delay;	/* in msecs, for IRQ-only buttons */
 
 	struct delayed_work work;
+	struct hrtimer debounce_timer;
 	unsigned int software_debounce;	/* in msecs, for GPIO-driven buttons */
 
 	unsigned int irq;
@@ -49,6 +50,7 @@ struct gpio_button_data {
 	bool disabled;
 	bool key_pressed;
 	bool suspended;
+	bool debounce_use_hrtimer;
 };
 
 struct gpio_keys_drvdata {
@@ -144,10 +146,12 @@ static void gpio_keys_disable_button(struct gpio_button_data *bdata)
 		 */
 		disable_irq(bdata->irq);
 
-		if (bdata->gpiod)
-			cancel_delayed_work_sync(&bdata->work);
-		else
+		if (!bdata->gpiod)
 			hrtimer_cancel(&bdata->release_timer);
+		else if (bdata->debounce_use_hrtimer)
+			hrtimer_cancel(&bdata->debounce_timer);
+		else
+			cancel_delayed_work_sync(&bdata->work);
 
 		bdata->disabled = true;
 	}
@@ -361,7 +365,10 @@ static void gpio_keys_gpio_report_event(struct gpio_button_data *bdata)
 	unsigned int type = button->type ?: EV_KEY;
 	int state;
 
-	state = gpiod_get_value_cansleep(bdata->gpiod);
+	if (bdata->debounce_use_hrtimer)
+		state = gpiod_get_value(bdata->gpiod);
+	else
+		state = gpiod_get_value_cansleep(bdata->gpiod);
 	if (state < 0) {
 		dev_err(input->dev.parent,
 			"failed to get gpio state: %d\n", state);
@@ -376,11 +383,8 @@ static void gpio_keys_gpio_report_event(struct gpio_button_data *bdata)
 	}
 }
 
-static void gpio_keys_gpio_work_func(struct work_struct *work)
+static void gpio_keys_debounce_event(struct gpio_button_data *bdata)
 {
-	struct gpio_button_data *bdata =
-		container_of(work, struct gpio_button_data, work.work);
-
 	gpio_keys_gpio_report_event(bdata);
 	input_sync(bdata->input);
 
@@ -388,6 +392,26 @@ static void gpio_keys_gpio_work_func(struct work_struct *work)
 		pm_relax(bdata->input->dev.parent);
 }
 
+static void gpio_keys_gpio_work_func(struct work_struct *work)
+{
+	struct gpio_button_data *bdata = container_of(work,
+						      struct gpio_button_data,
+						      work.work);
+
+	gpio_keys_debounce_event(bdata);
+}
+
+static enum hrtimer_restart gpio_keys_debounce_timer(struct hrtimer *t)
+{
+	struct gpio_button_data *bdata = container_of(t,
+						      struct gpio_button_data,
+						      debounce_timer);
+
+	gpio_keys_debounce_event(bdata);
+
+	return HRTIMER_NORESTART;
+}
+
 static irqreturn_t gpio_keys_gpio_isr(int irq, void *dev_id)
 {
 	struct gpio_button_data *bdata = dev_id;
@@ -409,9 +433,15 @@ static irqreturn_t gpio_keys_gpio_isr(int irq, void *dev_id)
 		}
 	}
 
-	mod_delayed_work(system_wq,
-			 &bdata->work,
-			 msecs_to_jiffies(bdata->software_debounce));
+	if (bdata->debounce_use_hrtimer) {
+		hrtimer_start(&bdata->debounce_timer,
+			      ms_to_ktime(bdata->software_debounce),
+			      HRTIMER_MODE_REL);
+	} else {
+		mod_delayed_work(system_wq,
+				 &bdata->work,
+				 msecs_to_jiffies(bdata->software_debounce));
+	}
 
 	return IRQ_HANDLED;
 }
@@ -471,10 +501,12 @@ static void gpio_keys_quiesce_key(void *data)
 {
 	struct gpio_button_data *bdata = data;
 
-	if (bdata->gpiod)
+	if (bdata->gpiod) {
+		hrtimer_cancel(&bdata->debounce_timer);
 		cancel_delayed_work_sync(&bdata->work);
-	else
+	} else {
 		hrtimer_cancel(&bdata->release_timer);
+	}
 }
 
 static int gpio_keys_setup_key(struct platform_device *pdev,
@@ -546,6 +578,13 @@ static int gpio_keys_setup_key(struct platform_device *pdev,
 			if (error < 0)
 				bdata->software_debounce =
 						button->debounce_interval;
+
+			/*
+			 * If reading the GPIO won't sleep, we can use a hrtimer
+			 * instead of a standard timer for the software
+			 * debounce, to reduce the latency as much as possible.
+			 */
+			bdata->debounce_use_hrtimer = !gpiod_cansleep(bdata->gpiod);
 		}
 
 		if (button->irq) {
@@ -564,6 +603,10 @@ static int gpio_keys_setup_key(struct platform_device *pdev,
 
 		INIT_DELAYED_WORK(&bdata->work, gpio_keys_gpio_work_func);
 
+		hrtimer_init(&bdata->debounce_timer,
+			     CLOCK_REALTIME, HRTIMER_MODE_REL);
+		bdata->debounce_timer.function = gpio_keys_debounce_timer;
+
 		isr = gpio_keys_gpio_isr;
 		irqflags = IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING;
 
-- 
2.30.1