rtc: brcmstb-waketimer: compensate for lack of wktmr disable

Since the WKTMR hardware block cannot be disabled it is necessary
for the driver to accommodate for associated timing hazards. This
commit targets the following possibilities:

A possible race between clearing a wktmr event and the alarm expiring
is made one-sided by setting the alarm to its maximum value before
clearing the event.

Programming alarm values close to the current time may not trigger
events if the counter advances while the alarm is being programmed.
After programming an alarm, a check is made to ensure that it is
either in the future or an expiration event is pending.
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230120190147.718976-4-opendmb@gmail.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-brcmstb-waketimer.c

@@ -31,6 +31,8 @@ struct brcmstb_waketmr {
 	struct notifier_block reboot_notifier;
 	struct clk *clk;
 	u32 rate;
+	unsigned long rtc_alarm;
+	bool alarm_en;
 };
 
 #define BRCMSTB_WKTMR_EVENT		0x00
@@ -52,6 +54,11 @@ static inline bool brcmstb_waketmr_is_pending(struct brcmstb_waketmr *timer)
 
 static inline void brcmstb_waketmr_clear_alarm(struct brcmstb_waketmr *timer)
 {
+	u32 reg;
+
+	timer->alarm_en = false;
+	reg = readl_relaxed(timer->base + BRCMSTB_WKTMR_COUNTER);
+	writel_relaxed(reg - 1, timer->base + BRCMSTB_WKTMR_ALARM);
 	writel_relaxed(WKTMR_ALARM_EVENT, timer->base + BRCMSTB_WKTMR_EVENT);
 	(void)readl_relaxed(timer->base + BRCMSTB_WKTMR_EVENT);
 }
@@ -59,12 +66,22 @@ static inline void brcmstb_waketmr_clear_alarm(struct brcmstb_waketmr *timer)
 static void brcmstb_waketmr_set_alarm(struct brcmstb_waketmr *timer,
 				      unsigned int secs)
 {
+	unsigned int now;
+
 	brcmstb_waketmr_clear_alarm(timer);
 
 	/* Make sure we are actually counting in seconds */
 	writel_relaxed(timer->rate, timer->base + BRCMSTB_WKTMR_PRESCALER);
 
-	writel_relaxed(secs + 1, timer->base + BRCMSTB_WKTMR_ALARM);
+	writel_relaxed(secs, timer->base + BRCMSTB_WKTMR_ALARM);
+	now = readl_relaxed(timer->base + BRCMSTB_WKTMR_COUNTER);
+
+	while ((int)(secs - now) <= 0 &&
+		!brcmstb_waketmr_is_pending(timer)) {
+		secs = now + 1;
+		writel_relaxed(secs, timer->base + BRCMSTB_WKTMR_ALARM);
+		now = readl_relaxed(timer->base + BRCMSTB_WKTMR_COUNTER);
+	}
 }
 
 static irqreturn_t brcmstb_waketmr_irq(int irq, void *data)
@@ -155,27 +172,30 @@ static int brcmstb_waketmr_getalarm(struct device *dev,
 				    struct rtc_wkalrm *alarm)
 {
 	struct brcmstb_waketmr *timer = dev_get_drvdata(dev);
-	time64_t sec;
 
-	sec = readl_relaxed(timer->base + BRCMSTB_WKTMR_ALARM);
-	if (sec != 0) {
-		/* Alarm is enabled */
-		alarm->enabled = 1;
-		rtc_time64_to_tm(sec, &alarm->time);
-	}
+	alarm->enabled = timer->alarm_en;
+	rtc_time64_to_tm(timer->rtc_alarm, &alarm->time);
 
 	alarm->pending = brcmstb_waketmr_is_pending(timer);
 
 	return 0;
 }
 
-/*
- * Does not do much but keep the RTC class happy. We always support
- * alarms.
- */
 static int brcmstb_waketmr_alarm_enable(struct device *dev,
 					unsigned int enabled)
 {
+	struct brcmstb_waketmr *timer = dev_get_drvdata(dev);
+
+	if (enabled && !timer->alarm_en) {
+		if ((int)(readl_relaxed(timer->base + BRCMSTB_WKTMR_COUNTER) -
+		    readl_relaxed(timer->base + BRCMSTB_WKTMR_ALARM)) >= 0 &&
+		    !brcmstb_waketmr_is_pending(timer))
+			return -EINVAL;
+		timer->alarm_en = true;
+	} else if (!enabled && timer->alarm_en) {
+		timer->alarm_en = false;
+	}
+
 	return 0;
 }
 
@@ -183,14 +203,10 @@ static int brcmstb_waketmr_setalarm(struct device *dev,
 				     struct rtc_wkalrm *alarm)
 {
 	struct brcmstb_waketmr *timer = dev_get_drvdata(dev);
-	time64_t sec;
 
-	if (alarm->enabled)
-		sec = rtc_tm_to_time64(&alarm->time);
-	else
-		sec = 0;
+	timer->rtc_alarm = rtc_tm_to_time64(&alarm->time);
 
-	brcmstb_waketmr_set_alarm(timer, sec);
+	brcmstb_waketmr_set_alarm(timer, timer->rtc_alarm);
 
 	return brcmstb_waketmr_alarm_enable(dev, alarm->enabled);
 }