rtc: snvs: Add timeouts to avoid kernel lockups

In order to read correctly from asynchronously updated RTC registers,
it's necessary to read repeatedly until their values do not change from
read to read.  It's also necessary to wait for three RTC clock ticks for
certain operations.  There are no timeouts in this code and these
operations could possibly loop forever.

To avoid kernel hangs, put in timeouts.

The iMX7d can be configured to stop the SRTC on a tamper event, which
will lockup the kernel inside this driver as described above.

These hangs can happen when running under qemu, which doesn't emulate
the SNVS RTC, though currently the driver will refuse to load on qemu
due to a timeout in the driver probe method.

It could also happen if the SRTC block where somehow placed into reset
or the slow speed clock that drives the SRTC counter (but not the CPU)
were to stop.

The symptoms on a two core iMX7d are a work queue hang on
rtc_timer_do_work(), which eventually blocks a systemd fsnotify
operation that triggers a work queue flush, causing systemd to hang and
thus causing all services that should be started by systemd, like a
console getty, to fail to start or stop.

Also optimize the wait code to wait less.  It only needs to wait for the
clock to advance three ticks, not to see it change three times.

Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-snvs.c

@@ -40,49 +40,83 @@ struct snvs_rtc_data {
 	struct clk *clk;
 };
 
+/* Read 64 bit timer register, which could be in inconsistent state */
+static u64 rtc_read_lpsrt(struct snvs_rtc_data *data)
+{
+	u32 msb, lsb;
+
+	regmap_read(data->regmap, data->offset + SNVS_LPSRTCMR, &msb);
+	regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &lsb);
+	return (u64)msb << 32 | lsb;
+}
+
+/* Read the secure real time counter, taking care to deal with the cases of the
+ * counter updating while being read.
+ */
 static u32 rtc_read_lp_counter(struct snvs_rtc_data *data)
 {
 	u64 read1, read2;
-	u32 val;
+	unsigned int timeout = 100;
 
+	/* As expected, the registers might update between the read of the LSB
+	 * reg and the MSB reg.  It's also possible that one register might be
+	 * in partially modified state as well.
+	 */
+	read1 = rtc_read_lpsrt(data);
 	do {
-		regmap_read(data->regmap, data->offset + SNVS_LPSRTCMR, &val);
-		read1 = val;
-		read1 <<= 32;
-		regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &val);
-		read1 |= val;
-
-		regmap_read(data->regmap, data->offset + SNVS_LPSRTCMR, &val);
-		read2 = val;
-		read2 <<= 32;
-		regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &val);
-		read2 |= val;
-	} while (read1 != read2);
+		read2 = read1;
+		read1 = rtc_read_lpsrt(data);
+	} while (read1 != read2 && --timeout);
+	if (!timeout)
+		dev_err(&data->rtc->dev, "Timeout trying to get valid LPSRT Counter read\n");
 
 	/* Convert 47-bit counter to 32-bit raw second count */
 	return (u32) (read1 >> CNTR_TO_SECS_SH);
 }
 
-static void rtc_write_sync_lp(struct snvs_rtc_data *data)
+/* Just read the lsb from the counter, dealing with inconsistent state */
+static int rtc_read_lp_counter_lsb(struct snvs_rtc_data *data, u32 *lsb)
+{
+	u32 count1, count2;
+	unsigned int timeout = 100;
+
+	regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count1);
+	do {
+		count2 = count1;
+		regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count1);
+	} while (count1 != count2 && --timeout);
+	if (!timeout) {
+		dev_err(&data->rtc->dev, "Timeout trying to get valid LPSRT Counter read\n");
+		return -ETIMEDOUT;
+	}
+
+	*lsb = count1;
+	return 0;
+}
+
+static int rtc_write_sync_lp(struct snvs_rtc_data *data)
 {
-	u32 count1, count2, count3;
-	int i;
-
-	/* Wait for 3 CKIL cycles */
-	for (i = 0; i < 3; i++) {
-		do {
-			regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count1);
-			regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count2);
-		} while (count1 != count2);
-
-		/* Now wait until counter value changes */
-		do {
-			do {
-				regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count2);
-				regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count3);
-			} while (count2 != count3);
-		} while (count3 == count1);
+	u32 count1, count2;
+	u32 elapsed;
+	unsigned int timeout = 1000;
+	int ret;
+
+	ret = rtc_read_lp_counter_lsb(data, &count1);
+	if (ret)
+		return ret;
+
+	/* Wait for 3 CKIL cycles, about 61.0-91.5 µs */
+	do {
+		ret = rtc_read_lp_counter_lsb(data, &count2);
+		if (ret)
+			return ret;
+		elapsed = count2 - count1; /* wrap around _is_ handled! */
+	} while (elapsed < 3 && --timeout);
+	if (!timeout) {
+		dev_err(&data->rtc->dev, "Timeout waiting for LPSRT Counter to change\n");
+		return -ETIMEDOUT;
 	}
+	return 0;
 }
 
 static int snvs_rtc_enable(struct snvs_rtc_data *data, bool enable)
@@ -166,9 +200,7 @@ static int snvs_rtc_alarm_irq_enable(struct device *dev, unsigned int enable)
 			   (SNVS_LPCR_LPTA_EN | SNVS_LPCR_LPWUI_EN),
 			   enable ? (SNVS_LPCR_LPTA_EN | SNVS_LPCR_LPWUI_EN) : 0);
 
-	rtc_write_sync_lp(data);
-
-	return 0;
+	return rtc_write_sync_lp(data);
 }
 
 static int snvs_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
@@ -176,11 +208,14 @@ static int snvs_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	struct snvs_rtc_data *data = dev_get_drvdata(dev);
 	struct rtc_time *alrm_tm = &alrm->time;
 	unsigned long time;
+	int ret;
 
 	rtc_tm_to_time(alrm_tm, &time);
 
 	regmap_update_bits(data->regmap, data->offset + SNVS_LPCR, SNVS_LPCR_LPTA_EN, 0);
-	rtc_write_sync_lp(data);
+	ret = rtc_write_sync_lp(data);
+	if (ret)
+		return ret;
 	regmap_write(data->regmap, data->offset + SNVS_LPTAR, time);
 
 	/* Clear alarm interrupt status bit */