rtc: pcf8523: set xtal load capacitance from DT

Add support for specifying the xtal load capacitance in the DT node.
The pcf8523 supports xtal load capacitance of 7pF or 12.5pF.
If the rtc has the wrong configuration the time will
drift several hours/week.

The driver use the default value 12.5pF.

The DT may specify either 7000fF or 12500fF.
(The DT uses femto Farad to avoid decimal numbers).
Other values are warned and the driver uses the default value.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

1 files changed, 20 insertions, 8 deletions

diff --git a/drivers/rtc/rtc-pcf8523.c b/drivers/rtc/rtc-pcf8523.c
index fb13933f4006..b5c61a70b5df 100644
--- a/drivers/rtc/rtc-pcf8523.c
+++ b/drivers/rtc/rtc-pcf8523.c
@@ -97,8 +97,9 @@ static int pcf8523_voltage_low(struct i2c_client *client)
         return !!(value & REG_CONTROL3_BLF);
 }
 
-static int pcf8523_select_capacitance(struct i2c_client *client, bool high)
+static int pcf8523_load_capacitance(struct i2c_client *client)
 {
+        u32 load;
         u8 value;
         int err;
 
@@ -106,14 +107,24 @@ static int pcf8523_select_capacitance(struct i2c_client *client, bool high)
         if (err < 0)
                 return err;
 
-        if (!high)
-                value &= ~REG_CONTROL1_CAP_SEL;
-        else
+        load = 12500;
+        of_property_read_u32(client->dev.of_node, "quartz-load-femtofarads",
+                             &load);
+
+        switch (load) {
+        default:
+                dev_warn(&client->dev, "Unknown quartz-load-femtofarads value: %d. Assuming 12500",
+                         load);
+                /* fall through */
+        case 12500:
                 value |= REG_CONTROL1_CAP_SEL;
+                break;
+        case 7000:
+                value &= ~REG_CONTROL1_CAP_SEL;
+                break;
+        }
 
         err = pcf8523_write(client, REG_CONTROL1, value);
-        if (err < 0)
-                return err;
 
         return err;
 }
@@ -347,9 +358,10 @@ static int pcf8523_probe(struct i2c_client *client,
         if (!pcf)
                 return -ENOMEM;
 
-        err = pcf8523_select_capacitance(client, true);
+        err = pcf8523_load_capacitance(client);
         if (err < 0)
-                return err;
+                dev_warn(&client->dev, "failed to set xtal load capacitance: %d",
+                         err);
 
         err = pcf8523_set_pm(client, 0);
         if (err < 0)