[PATCH v1 2/2] rtc: bq32000: add configurable delay between RTC reads

[Adriana Stancu <adriana@arista.com>]

When the RTC is used on systems without a interrupt line, userspace
tools like `hwclock` fall back to a frequent polling loop to synchronize
with the edge of the next second.

On the BQ32000, this aggressive polling can temporarly lock the register
refresh cycle, because the continuous transfers prevent the hardware from
updating the buffer. This results in stale data reads or select() timeouts
in userspace.

This patch introduces a configurable settle delay via `ti,read-settle-us`
property. If this property is specified, the driver uses a delay before
reading the RTC registers. This provides a sufficient idle time for the
hardware to sync with the register buffer.

Signed-off-by: Adriana Stancu <adriana@arista.com>
---
 drivers/rtc/rtc-bq32k.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/drivers/rtc/rtc-bq32k.c b/drivers/rtc/rtc-bq32k.c
index 7ad34539be4d..0cbfa0909732 100644
--- a/drivers/rtc/rtc-bq32k.c
+++ b/drivers/rtc/rtc-bq32k.c
@@ -16,6 +16,7 @@
 #include <linux/kstrtox.h>
 #include <linux/errno.h>
 #include <linux/bcd.h>
+#include <linux/delay.h>
 
 #define BQ32K_SECONDS		0x00	/* Seconds register address */
 #define BQ32K_SECONDS_MASK	0x7F	/* Mask over seconds value */
@@ -48,6 +49,11 @@ struct bq32k_regs {
 	uint8_t		years;
 };
 
+struct bq32k_data {
+	struct rtc_device *rtc;
+	u32 read_delay_us;
+};
+
 static struct i2c_driver bq32k_driver;
 
 static int bq32k_read(struct device *dev, void *data, uint8_t off, uint8_t len)
@@ -89,9 +95,17 @@ static int bq32k_write(struct device *dev, void *data, uint8_t off, uint8_t len)
 
 static int bq32k_rtc_read_time(struct device *dev, struct rtc_time *tm)
 {
+	struct bq32k_data *bq32k = dev_get_drvdata(dev);
 	struct bq32k_regs regs;
 	int error;
 
+	/*
+	 * When the device doesn't have the interrupt connected, prevent
+	 * userpace from polling the RTC registers to frequently.
+	 */
+	if (bq32k && bq32k->read_delay_us)
+		usleep_range(bq32k->read_delay_us, bq32k->read_delay_us + 50);
+
 	error = bq32k_read(dev, &regs, 0, sizeof(regs));
 	if (error)
 		return error;
@@ -253,13 +267,18 @@ static void bq32k_sysfs_unregister(struct device *dev)
 static int bq32k_probe(struct i2c_client *client)
 {
 	struct device *dev = &client->dev;
-	struct rtc_device *rtc;
+	struct bq32k_data *bq32k;
 	uint8_t reg;
 	int error;
+	uint32_t settle_us = 0;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
 		return -ENODEV;
 
+	bq32k = devm_kzalloc(dev, sizeof(*bq32k), GFP_KERNEL);
+	if (!bq32k)
+		return -ENOMEM;
+
 	/* Check Oscillator Stop flag */
 	error = bq32k_read(dev, &reg, BQ32K_SECONDS, 1);
 	if (!error && (reg & BQ32K_STOP)) {
@@ -280,10 +299,13 @@ static int bq32k_probe(struct i2c_client *client)
 	if (client->dev.of_node)
 		trickle_charger_of_init(dev, client->dev.of_node);
 
-	rtc = devm_rtc_device_register(&client->dev, bq32k_driver.driver.name,
-						&bq32k_rtc_ops, THIS_MODULE);
-	if (IS_ERR(rtc))
-		return PTR_ERR(rtc);
+	bq32k->rtc = devm_rtc_device_register(&client->dev, bq32k_driver.driver.name,
+					      &bq32k_rtc_ops, THIS_MODULE);
+	if (IS_ERR(bq32k->rtc))
+		return PTR_ERR(bq32k->rtc);
+
+	device_property_read_u32(dev, "ti,read-settle-us", &settle_us);
+	bq32k->read_delay_us = settle_us;
 
 	error = bq32k_sysfs_register(&client->dev);
 	if (error) {
@@ -293,7 +315,7 @@ static int bq32k_probe(struct i2c_client *client)
 	}
 
 
-	i2c_set_clientdata(client, rtc);
+	i2c_set_clientdata(client, bq32k);
 
 	return 0;
 }
-- 
2.51.0