clocksource/drivers/stm32-lptimer: Add support for suspend / resume

Add support for power management on STM32 LPTIMER clocksource driver:
- Upon clockevents_suspend(), shutdown the LPTIMER, and balance the
clk_prepare_enable() from the probe routine.
- Upon clockevents_resume(), restore the prescaler that may have been lost
during low power mode, and restore the clock.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20250224172101.3448398-1-fabrice.gasnier@foss.st.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Author: Fabrice Gasnier

drivers/clocksource/timer-stm32-lp.c

@@ -24,7 +24,9 @@ struct stm32_lp_private {
 	struct regmap *reg;
 	struct clock_event_device clkevt;
 	unsigned long period;
+	u32 psc;
 	struct device *dev;
+	struct clk *clk;
 };
 
 static struct stm32_lp_private*
@@ -120,6 +122,27 @@ static void stm32_clkevent_lp_set_prescaler(struct stm32_lp_private *priv,
 	/* Adjust rate and period given the prescaler value */
 	*rate = DIV_ROUND_CLOSEST(*rate, (1 << i));
 	priv->period = DIV_ROUND_UP(*rate, HZ);
+	priv->psc = i;
+}
+
+static void stm32_clkevent_lp_suspend(struct clock_event_device *clkevt)
+{
+	struct stm32_lp_private *priv = to_priv(clkevt);
+
+	stm32_clkevent_lp_shutdown(clkevt);
+
+	/* balance clk_prepare_enable() from the probe */
+	clk_disable_unprepare(priv->clk);
+}
+
+static void stm32_clkevent_lp_resume(struct clock_event_device *clkevt)
+{
+	struct stm32_lp_private *priv = to_priv(clkevt);
+
+	clk_prepare_enable(priv->clk);
+
+	/* restore prescaler */
+	regmap_write(priv->reg, STM32_LPTIM_CFGR, priv->psc << CFGR_PSC_OFFSET);
 }
 
 static void stm32_clkevent_lp_init(struct stm32_lp_private *priv,
@@ -134,6 +157,8 @@ static void stm32_clkevent_lp_init(struct stm32_lp_private *priv,
 	priv->clkevt.set_state_oneshot = stm32_clkevent_lp_set_oneshot;
 	priv->clkevt.set_next_event = stm32_clkevent_lp_set_next_event;
 	priv->clkevt.rating = STM32_LP_RATING;
+	priv->clkevt.suspend = stm32_clkevent_lp_suspend;
+	priv->clkevt.resume = stm32_clkevent_lp_resume;
 
 	clockevents_config_and_register(&priv->clkevt, rate, 0x1,
 					STM32_LPTIM_MAX_ARR);
@@ -151,11 +176,12 @@ static int stm32_clkevent_lp_probe(struct platform_device *pdev)
 		return -ENOMEM;
 
 	priv->reg = ddata->regmap;
-	ret = clk_prepare_enable(ddata->clk);
+	priv->clk = ddata->clk;
+	ret = clk_prepare_enable(priv->clk);
 	if (ret)
 		return -EINVAL;
 
-	rate = clk_get_rate(ddata->clk);
+	rate = clk_get_rate(priv->clk);
 	if (!rate) {
 		ret = -EINVAL;
 		goto out_clk_disable;
@@ -191,7 +217,7 @@ static int stm32_clkevent_lp_probe(struct platform_device *pdev)
 	return 0;
 
 out_clk_disable:
-	clk_disable_unprepare(ddata->clk);
+	clk_disable_unprepare(priv->clk);
 	return ret;
 }