[PATCH v2 0/3] rtc: Set M41T82 & M41T83 xtal load capacitance from DT

[PATCH v2 0/3] rtc: Set M41T82 & M41T83 xtal load capacitance from DT

[Dennis Lambe Jr]

I haven't heard any feedback on this series aside from the dt-bindings,
but with the holidays I'm not assuming that means they're perfect. I
don't want to sit on the fixes I've already incorporated any longer
though, so here's v2.

Changes in v2:
* dt-bindings: remove accidental wakeup-sources line
    suggested by Krzysztof Kozlowski
* spelling fixes in changelogs

The m41t82 and m41t83 have an adjustable internal capacitance that
defaults to 25 pF per xtal pin. This patch series adds the ability to
configure it via the devicetree.

Patch 1 just changes `#ifdef CONFIG_OF` to `if (IS_ENABLED(CONFIG_OF))`
in m41t80_probe() so that I don't need to use __maybe_unused on my new
functions and variables.

Patch 2 is the dt-bindings.

Patch 3 is the actual feature implementation.

The desired capacitance comes from the quartz-load-femtofarads property,
following the example of two other RTC ICs that have adjustable internal
load capacitance, the NXP pcf85063 and pcf8523. The m41t82 and m41t83
support much finer-grained control over the capacitance than those
chips, and ST calls the feature "analog calibration", but it looks to me
like it's essentially the same kind of thing.

My use case for this is:

ST specifies not to add any additional external load capacitance[1], but
the MikroElektronika RTC 9 Click board[2] has a 22 pF cap on each xtal
pin[3]. The resulting combined capacitance appears to be outside of the
operating range of the xtal, because when power is removed from the
boards I'm testing with, the RTC reports an Oscillator-Fail flag on the
next power on.

I found I could work around the problem by reducing the internal load
capacitance as low as it will go.

References:
[1] https://www.st.com/resource/en/application_note/an3060-applications-guide-for-serial-realtime-clocks-rtcs-stmicroelectronics.pdf
[2] https://www.mikroe.com/rtc-9-click
[3] https://download.mikroe.com/documents/add-on-boards/click/rtc-9/rtc-9-click-schematic-v100.pdf

Previous versions:
v1: https://lore.kernel.org/linux-rtc/20221219190915.3912384-1-dennis@sparkcharge.io/T/

Dennis Lambe Jr (3):
  rtc: m41t80: probe: use IS_ENABLED for CONFIG_OF
  dt-bindings: m41t80: add xtal load capacitance
  rtc: m41t80: set xtal load capacitance from DT

 .../devicetree/bindings/rtc/st,m41t80.yaml    | 16 ++++
 drivers/rtc/rtc-m41t80.c                      | 84 +++++++++++++++++--
 2 files changed, 92 insertions(+), 8 deletions(-)

-- 
2.25.1

[PATCH v2 1/3] rtc: m41t80: probe: use IS_ENABLED for CONFIG_OF

[Dennis Lambe Jr]

The style guide recommends IS_ENABLED rather than ifdef for wrapping
conditional code wherever possible.

Functions that are only called on DeviceTree platforms would otherwise
need to be cluttered up with __maybe_unused, which is especially
undesirable if there's nothing inherently DT-specific about those
functions.

Signed-off-by: Dennis Lambe Jr <dennis@sparkcharge.io>
---

Notes:
    v1 -> v2: spelling fix in changelog

 drivers/rtc/rtc-m41t80.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-m41t80.c b/drivers/rtc/rtc-m41t80.c
index 494052dbd39f..f963b76e5fc0 100644
--- a/drivers/rtc/rtc-m41t80.c
+++ b/drivers/rtc/rtc-m41t80.c
@@ -909,10 +909,11 @@ static int m41t80_probe(struct i2c_client *client)
 	if (IS_ERR(m41t80_data->rtc))
 		return PTR_ERR(m41t80_data->rtc);
 
-#ifdef CONFIG_OF
-	wakeup_source = of_property_read_bool(client->dev.of_node,
-					      "wakeup-source");
-#endif
+	if (IS_ENABLED(CONFIG_OF)) {
+		wakeup_source = of_property_read_bool(client->dev.of_node,
+						      "wakeup-source");
+	}
+
 	if (client->irq > 0) {
 		rc = devm_request_threaded_irq(&client->dev, client->irq,
 					       NULL, m41t80_handle_irq,
-- 
2.25.1

[PATCH v2 2/3] dt-bindings: m41t80: add xtal load capacitance

[Dennis Lambe Jr]

The ST m41t82 and m41t83 support programmable load capacitance from 3.5
pF to 17.4 pF. The hardware defaults to 12.5 pF.

The accuracy of the xtal can be calibrated precisely by adjusting the
load capacitance.

Add default, minimum, and maximum for the standard rtc property
quartz-load-femtofarads on compatible devices.

Signed-off-by: Dennis Lambe Jr <dennis@sparkcharge.io>
---

Notes:
    v1 -> v2:
    	remove accidental wakeup-sources line
    		suggested by Krzysztof Kozlowski
    	spelling fix in changelog

 .../devicetree/bindings/rtc/st,m41t80.yaml       | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/devicetree/bindings/rtc/st,m41t80.yaml b/Documentation/devicetree/bindings/rtc/st,m41t80.yaml
index fc9c6da6483f..6673adf6e99b 100644
--- a/Documentation/devicetree/bindings/rtc/st,m41t80.yaml
+++ b/Documentation/devicetree/bindings/rtc/st,m41t80.yaml
@@ -33,6 +33,11 @@ properties:
   "#clock-cells":
     const: 1
 
+  quartz-load-femtofarads:
+    default: 12500
+    minimum: 3500
+    maximum: 17375
+
   clock-output-names:
     maxItems: 1
     description: From common clock binding to override the default output clock name.
@@ -46,6 +51,17 @@ properties:
 
 allOf:
   - $ref: rtc.yaml
+  - if:
+      not:
+        properties:
+          compatible:
+            contains:
+              enum:
+                - st,m41t82
+                - st,m41t83
+    then:
+      properties:
+        quartz-load-femtofarads: false
 
 unevaluatedProperties: false
 
-- 
2.25.1

Re: [PATCH v2 2/3] dt-bindings: m41t80: add xtal load capacitance

[Krzysztof Kozlowski]

On 04/01/2023 06:25, Dennis Lambe Jr wrote:
> The ST m41t82 and m41t83 support programmable load capacitance from 3.5
> pF to 17.4 pF. The hardware defaults to 12.5 pF.
> 
> The accuracy of the xtal can be calibrated precisely by adjusting the
> load capacitance.
> 
> Add default, minimum, and maximum for the standard rtc property
> quartz-load-femtofarads on compatible devices.
> 
> Signed-off-by: Dennis Lambe Jr <dennis@sparkcharge.io>
> ---
> 


Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Best regards,
Krzysztof

[PATCH v2 3/3] rtc: m41t80: set xtal load capacitance from DT

[Dennis Lambe Jr]

Add support for specifying the xtal load capacitance in the DT node for
devices with an Analog Calibration register.

the m41t82 and m41t83 support xtal load capacitance from 3.5 pF to 17.4
pF.

If no xtal load capacitance is specified, the battery-backed register
won't be modified. The hardware defaults to 12.5 pF on reset.

Signed-off-by: Dennis Lambe Jr <dennis@sparkcharge.io>
---
 drivers/rtc/rtc-m41t80.c | 75 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 71 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-m41t80.c b/drivers/rtc/rtc-m41t80.c
index f963b76e5fc0..85bde7130a4d 100644
--- a/drivers/rtc/rtc-m41t80.c
+++ b/drivers/rtc/rtc-m41t80.c
@@ -44,12 +44,17 @@
 #define M41T80_REG_ALARM_MIN	0x0d
 #define M41T80_REG_ALARM_SEC	0x0e
 #define M41T80_REG_FLAGS	0x0f
+#define M41T80_REG_AC		0x12
 #define M41T80_REG_SQW		0x13
 
 #define M41T80_DATETIME_REG_SIZE	(M41T80_REG_YEAR + 1)
 #define M41T80_ALARM_REG_SIZE	\
 	(M41T80_REG_ALARM_SEC + 1 - M41T80_REG_ALARM_MON)
 
+#define M41T80_AC_MIN		 3500
+#define M41T80_AC_MAX		17375
+#define M41T80_AC_DEFAULT	12500
+
 #define M41T80_SQW_MAX_FREQ	32768
 
 #define M41T80_SEC_ST		BIT(7)	/* ST: Stop Bit */
@@ -68,6 +73,7 @@
 #define M41T80_FEATURE_SQ	BIT(2)	/* Squarewave feature */
 #define M41T80_FEATURE_WD	BIT(3)	/* Extra watchdog resolution */
 #define M41T80_FEATURE_SQ_ALT	BIT(4)	/* RSx bits are in reg 4 */
+#define M41T80_FEATURE_AC	BIT(5) /* Analog calibration */
 
 static const struct i2c_device_id m41t80_id[] = {
 	{ "m41t62", M41T80_FEATURE_SQ | M41T80_FEATURE_SQ_ALT },
@@ -75,8 +81,10 @@ static const struct i2c_device_id m41t80_id[] = {
 	{ "m41t80", M41T80_FEATURE_SQ },
 	{ "m41t81", M41T80_FEATURE_HT | M41T80_FEATURE_SQ},
 	{ "m41t81s", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
-	{ "m41t82", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
-	{ "m41t83", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
+	{ "m41t82", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ
+		    | M41T80_FEATURE_AC },
+	{ "m41t83", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ
+		    | M41T80_FEATURE_AC },
 	{ "m41st84", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
 	{ "m41st85", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
 	{ "m41st87", M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ },
@@ -108,11 +116,13 @@ static const __maybe_unused struct of_device_id m41t80_of_match[] = {
 	},
 	{
 		.compatible = "st,m41t82",
-		.data = (void *)(M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ)
+		.data = (void *)(M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ
+				 | M41T80_FEATURE_AC)
 	},
 	{
 		.compatible = "st,m41t83",
-		.data = (void *)(M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ)
+		.data = (void *)(M41T80_FEATURE_HT | M41T80_FEATURE_BL | M41T80_FEATURE_SQ
+				 | M41T80_FEATURE_AC)
 	},
 	{
 		.compatible = "st,m41t84",
@@ -405,6 +415,54 @@ static const struct rtc_class_ops m41t80_rtc_ops = {
 	.alarm_irq_enable = m41t80_alarm_irq_enable,
 };
 
+static u8 to_sign_magnitude_u8(int n)
+{
+	if (n < 0)
+		return 0x80 | -n;
+	return n;
+}
+
+static int m41t80_encode_ac(int quartz_load)
+{
+	if (quartz_load < M41T80_AC_MIN || quartz_load > M41T80_AC_MAX)
+		return -EINVAL;
+
+	/*
+	 * register representation is the per-capacitor offset from its default
+	 * value in units of 1/4 pF, in sign-magnitude form.
+	 */
+	return to_sign_magnitude_u8((quartz_load - M41T80_AC_DEFAULT) / 125);
+}
+
+static int m41t80_set_ac(struct m41t80_data *m41t80_data, int quartz_load)
+{
+	struct i2c_client *client = m41t80_data->client;
+	struct device *dev = &client->dev;
+	int ret;
+	int ac;
+
+	if (!(m41t80_data->features & M41T80_FEATURE_AC)) {
+		dev_err(dev, "analog calibration requested but not supported\n");
+		return -EOPNOTSUPP;
+	}
+
+	ac = m41t80_encode_ac(quartz_load);
+	if (ac < 0) {
+		dev_err(dev, "quartz load %d fF out of range\n",
+			quartz_load);
+		return ac;
+	}
+
+	ret = i2c_smbus_write_byte_data(client, M41T80_REG_AC, ac);
+	if (ret < 0) {
+		dev_err(dev, "Can't set AC register\n");
+		return ret;
+	}
+
+	dev_info(dev, "quartz load set to %d fF (AC=0x%x)\n", quartz_load, ac);
+	return 0;
+}
+
 #ifdef CONFIG_PM_SLEEP
 static int m41t80_suspend(struct device *dev)
 {
@@ -883,6 +941,7 @@ static int m41t80_probe(struct i2c_client *client)
 	struct rtc_time tm;
 	struct m41t80_data *m41t80_data = NULL;
 	bool wakeup_source = false;
+	u32 quartz_load = M41T80_AC_DEFAULT;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_I2C_BLOCK |
 				     I2C_FUNC_SMBUS_BYTE_DATA)) {
@@ -912,6 +971,14 @@ static int m41t80_probe(struct i2c_client *client)
 	if (IS_ENABLED(CONFIG_OF)) {
 		wakeup_source = of_property_read_bool(client->dev.of_node,
 						      "wakeup-source");
+
+		rc = of_property_read_u32(client->dev.of_node,
+					  "quartz-load-femtofarads",
+					  &quartz_load);
+		if (!rc)
+			m41t80_set_ac(m41t80_data, quartz_load);
+		else if (rc != -EINVAL)
+			dev_err(&client->dev, "quartz-load-femtofarads property value is missing or invalid\n");
 	}
 
 	if (client->irq > 0) {
-- 
2.25.1