[PATCH 0/7] rtc: m41t93: add new features alarm, clock out, watchdog

[PATCH 0/7] rtc: m41t93: add new features alarm, clock out, watchdog

[Akhilesh Patil]

This patch series adds following to m41t93 rtc driver.

Functionalities: 
- Alarm support (support to configure alarm 1)
- Square wave output support
- Watchdog support

Code improvements:
this series migrates existing driver to use standard regmap interface
for spi instead of direct spi calls and uses regmap for new features.

Bug fix: 
Fixes device probe issue after power-on due to incorrect assumptions of
reset values of the registers.

Device tree support:
Adds device tree support to the driver along with binding documentation.

Testing:
This patch series is validated on TI am62x board with m41t93 rtc chip
connected to spi0 bus.
regmap migration is additionally tested by observing spi transfers
with the help of logic analyzer. Short summary of test flow is added in
commit message of respective features.

Datasheet:
https://www.st.com/resource/en/datasheet/m41t93.pdf

patch 4 to 7 depend on patch 3 (regmap patch)

Regards,
Akhilesh

Akhilesh Patil (7):
  rtc: m41t93: add device tree support
  dt-bindings: rtc: add bindings for m41t93
  rtc: m41t93: migrate to regmap api for register access
  rtc: m41t93: Add alarm support
  rtc: m41t93: fix device connection/detection logic during probe
  rtc: m41t93: Add square wave clock provider support
  rtc: m41t93: Add watchdog support

 .../devicetree/bindings/rtc/st,m41t93.yaml    |  43 ++
 drivers/rtc/rtc-m41t93.c                      | 489 ++++++++++++++++--
 2 files changed, 479 insertions(+), 53 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/rtc/st,m41t93.yaml

-- 
2.34.1

[PATCH 1/7] rtc: m41t93: add device tree support

[Akhilesh Patil]

Add device tree support for m41t93 rtc by adding of_match_table.
Define compatible string - "st,m41t93" which can be used to instantiate
this rtc device via DT node.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index 9444cb5f5190..4e803ff0ce49 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -191,9 +191,16 @@ static int m41t93_probe(struct spi_device *spi)
 	return 0;
 }
 
+static const struct of_device_id m41t93_dt_match[] = {
+	{ .compatible = "st,m41t93" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, m41t93_dt_match);
+
 static struct spi_driver m41t93_driver = {
 	.driver = {
 		.name	= "rtc-m41t93",
+		.of_match_table = m41t93_dt_match,
 	},
 	.probe	= m41t93_probe,
 };
-- 
2.34.1

[PATCH 2/7] dt-bindings: rtc: add bindings for m41t93

[Akhilesh Patil]

add DT bindings for m41t93 rtc in YAML format.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 .../devicetree/bindings/rtc/st,m41t93.yaml    | 43 +++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/rtc/st,m41t93.yaml

diff --git a/Documentation/devicetree/bindings/rtc/st,m41t93.yaml b/Documentation/devicetree/bindings/rtc/st,m41t93.yaml
new file mode 100644
index 000000000000..03673adc79db
--- /dev/null
+++ b/Documentation/devicetree/bindings/rtc/st,m41t93.yaml
@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/st,m41t93.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ST M41T93 RTC and compatible
+
+maintainers:
+  - Alexandre Belloni <alexandre.belloni@bootlin.com>
+
+properties:
+  compatible:
+    enum:
+      - st,m41t93
+
+  reg:
+    maxItems: 1
+
+  "#clock-cells":
+    const: 1
+
+allOf:
+  - $ref: rtc.yaml
+
+unevaluatedProperties: false
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      rtc@0 {
+        spi-max-frequency = <2000000>;
+        compatible = "st,m41t93";
+        reg = <0>;
+      };
+    };
+
-- 
2.34.1

Re: [PATCH 2/7] dt-bindings: rtc: add bindings for m41t93

[Krzysztof Kozlowski]

On 03/09/2025 16:25, Akhilesh Patil wrote:
> add DT bindings for m41t93 rtc in YAML format.

A nit, subject: drop second/last, redundant "bindings". The
"dt-bindings" prefix is already stating that these are bindings.
See also:
https://elixir.bootlin.com/linux/v6.17-rc3/source/Documentation/devicetree/bindings/submitting-patches.rst#L18


Please organize the patch documenting compatible (DT bindings) before
their user.
See also:
https://elixir.bootlin.com/linux/v6.14-rc6/source/Documentation/devicetree/bindings/submitting-patches.rst#L46

> 
> Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
> ---
>  .../devicetree/bindings/rtc/st,m41t93.yaml    | 43 +++++++++++++++++++
>  1 file changed, 43 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/rtc/st,m41t93.yaml
> 
> diff --git a/Documentation/devicetree/bindings/rtc/st,m41t93.yaml b/Documentation/devicetree/bindings/rtc/st,m41t93.yaml
> new file mode 100644
> index 000000000000..03673adc79db
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/rtc/st,m41t93.yaml
> @@ -0,0 +1,43 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/rtc/st,m41t93.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: ST M41T93 RTC and compatible
> +
> +maintainers:
> +  - Alexandre Belloni <alexandre.belloni@bootlin.com>

This should not be subsystem maintainer.

> +
> +properties:
> +  compatible:
> +    enum:
> +      - st,m41t93
> +
> +  reg:
> +    maxItems: 1
> +
> +  "#clock-cells":

Hm? Are you sure? Nothing in the driver nor commit msg suggests that.

> +    const: 1
> +
> +allOf:
> +  - $ref: rtc.yaml
> +
> +unevaluatedProperties: false

This goes after required. See example schema.

> +
> +required:
> +  - compatible
> +  - reg
> +
> +examples:
> +  - |
> +    spi {
> +      #address-cells = <1>;
> +      #size-cells = <0>;
> +      rtc@0 {
> +        spi-max-frequency = <2000000>;

Does not look tested.

> +        compatible = "st,m41t93";
> +        reg = <0>;

Please follow DTS coding style. Which property is the first in the
coding style?

Best regards,
Krzysztof

Re: [PATCH 2/7] dt-bindings: rtc: add bindings for m41t93

[Rob Herring (Arm)]

On Wed, 03 Sep 2025 19:55:59 +0530, Akhilesh Patil wrote:
> add DT bindings for m41t93 rtc in YAML format.
> 
> Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
> ---
>  .../devicetree/bindings/rtc/st,m41t93.yaml    | 43 +++++++++++++++++++
>  1 file changed, 43 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/rtc/st,m41t93.yaml
> 

My bot found errors running 'make dt_binding_check' on your patch:

yamllint warnings/errors:

dtschema/dtc warnings/errors:
/builds/robherring/dt-review-ci/linux/Documentation/devicetree/bindings/rtc/st,m41t93.example.dtb: rtc@0 (st,m41t93): Unevaluated properties are not allowed ('spi-max-frequency' was unexpected)
	from schema $id: http://devicetree.org/schemas/rtc/st,m41t93.yaml#

doc reference errors (make refcheckdocs):

See https://patchwork.ozlabs.org/project/devicetree-bindings/patch/40c91cbb07140ecdf4f91afc118c2518e85041c3.1756908788.git.akhilesh@ee.iitb.ac.in

The base for the series is generally the latest rc1. A different dependency
should be noted in *this* patch.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit after running the above command yourself. Note
that DT_SCHEMA_FILES can be set to your schema file to speed up checking
your schema. However, it must be unset to test all examples with your schema.

[PATCH 3/7] rtc: m41t93: migrate to regmap api for register access

[Akhilesh Patil]

Adapt driver to use regmap api with spi bus instead of direct spi
subsystem calls to access device registers. Simplify and standardize the
register interactions using more abstract and bus agnostic regmap api
to reduce code duplication and improve maintainability. Define spi regmap
config suitable for m41t93 spi bus protocol to achieve same transactions on
spi bus.

Tested on TI am62x sk board with m41t93 rtc chip connected over spi0.
Validated set and get time using hwclock tool and verified spi bus
transfers using logic analyzer.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 121 ++++++++++++++++++++++-----------------
 1 file changed, 70 insertions(+), 51 deletions(-)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index 4e803ff0ce49..ad862bf706b6 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -12,6 +12,7 @@
 #include <linux/platform_device.h>
 #include <linux/rtc.h>
 #include <linux/spi/spi.h>
+#include <linux/regmap.h>
 
 #define M41T93_REG_SSEC			0
 #define M41T93_REG_ST_SEC		1
@@ -31,23 +32,17 @@
 #define M41T93_FLAG_BL			(1 << 4)
 #define M41T93_FLAG_HT			(1 << 6)
 
-static inline int m41t93_set_reg(struct spi_device *spi, u8 addr, u8 data)
-{
-	u8 buf[2];
-
-	/* MSB must be '1' to write */
-	buf[0] = addr | 0x80;
-	buf[1] = data;
-
-	return spi_write(spi, buf, sizeof(buf));
-}
+struct m41t93_data {
+	struct rtc_device *rtc;
+	struct regmap *regmap;
+};
 
 static int m41t93_set_time(struct device *dev, struct rtc_time *tm)
 {
-	struct spi_device *spi = to_spi_device(dev);
-	int tmp;
-	u8 buf[9] = {0x80};        /* write cmd + 8 data bytes */
-	u8 * const data = &buf[1]; /* ptr to first data byte */
+	struct m41t93_data *m41t93 = dev_get_drvdata(dev);
+	int tmp, ret;
+	u8 buf[8] = {0};        /* 8 data bytes */
+	u8 * const data = &buf[0]; /* ptr to first data byte */
 
 	dev_dbg(dev, "%s secs=%d, mins=%d, "
 		"hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
@@ -56,31 +51,31 @@ static int m41t93_set_time(struct device *dev, struct rtc_time *tm)
 		tm->tm_mon, tm->tm_year, tm->tm_wday);
 
 	if (tm->tm_year < 100) {
-		dev_warn(&spi->dev, "unsupported date (before 2000-01-01).\n");
+		dev_warn(dev, "unsupported date (before 2000-01-01).\n");
 		return -EINVAL;
 	}
 
-	tmp = spi_w8r8(spi, M41T93_REG_FLAGS);
-	if (tmp < 0)
-		return tmp;
+	ret = regmap_read(m41t93->regmap, M41T93_REG_FLAGS, &tmp);
+	if (ret < 0)
+		return ret;
 
 	if (tmp & M41T93_FLAG_OF) {
-		dev_warn(&spi->dev, "OF bit is set, resetting.\n");
-		m41t93_set_reg(spi, M41T93_REG_FLAGS, tmp & ~M41T93_FLAG_OF);
+		dev_warn(dev, "OF bit is set, resetting.\n");
+		regmap_write(m41t93->regmap, M41T93_REG_FLAGS, tmp & ~M41T93_FLAG_OF);
 
-		tmp = spi_w8r8(spi, M41T93_REG_FLAGS);
-		if (tmp < 0) {
-			return tmp;
+		ret = regmap_read(m41t93->regmap, M41T93_REG_FLAGS, &tmp);
+		if (ret < 0) {
+			return ret;
 		} else if (tmp & M41T93_FLAG_OF) {
 			/* OF cannot be immediately reset: oscillator has to be
 			 * restarted. */
 			u8 reset_osc = buf[M41T93_REG_ST_SEC] | M41T93_FLAG_ST;
 
-			dev_warn(&spi->dev,
+			dev_warn(dev,
 				 "OF bit is still set, kickstarting clock.\n");
-			m41t93_set_reg(spi, M41T93_REG_ST_SEC, reset_osc);
+			regmap_write(m41t93->regmap, M41T93_REG_ST_SEC, reset_osc);
 			reset_osc &= ~M41T93_FLAG_ST;
-			m41t93_set_reg(spi, M41T93_REG_ST_SEC, reset_osc);
+			regmap_write(m41t93->regmap, M41T93_REG_ST_SEC, reset_osc);
 		}
 	}
 
@@ -94,14 +89,13 @@ static int m41t93_set_time(struct device *dev, struct rtc_time *tm)
 	data[M41T93_REG_MON]		= bin2bcd(tm->tm_mon + 1);
 	data[M41T93_REG_YEAR]		= bin2bcd(tm->tm_year % 100);
 
-	return spi_write(spi, buf, sizeof(buf));
+	return regmap_bulk_write(m41t93->regmap, M41T93_REG_SSEC, buf, sizeof(buf));
 }
 
 
 static int m41t93_get_time(struct device *dev, struct rtc_time *tm)
 {
-	struct spi_device *spi = to_spi_device(dev);
-	const u8 start_addr = 0;
+	struct m41t93_data *m41t93 = dev_get_drvdata(dev);
 	u8 buf[8];
 	int century_after_1900;
 	int tmp;
@@ -113,32 +107,32 @@ static int m41t93_get_time(struct device *dev, struct rtc_time *tm)
 	      case after poweron. Time is valid after resetting HT bit.
 	   2. oscillator fail bit (OF) is set: time is invalid.
 	*/
-	tmp = spi_w8r8(spi, M41T93_REG_ALM_HOUR_HT);
-	if (tmp < 0)
-		return tmp;
+	ret = regmap_read(m41t93->regmap, M41T93_REG_ALM_HOUR_HT, &tmp);
+	if (ret < 0)
+		return ret;
 
 	if (tmp & M41T93_FLAG_HT) {
-		dev_dbg(&spi->dev, "HT bit is set, reenable clock update.\n");
-		m41t93_set_reg(spi, M41T93_REG_ALM_HOUR_HT,
-			       tmp & ~M41T93_FLAG_HT);
+		dev_dbg(dev, "HT bit is set, reenable clock update.\n");
+		regmap_write(m41t93->regmap, M41T93_REG_ALM_HOUR_HT,
+			     tmp & ~M41T93_FLAG_HT);
 	}
 
-	tmp = spi_w8r8(spi, M41T93_REG_FLAGS);
-	if (tmp < 0)
-		return tmp;
+	ret = regmap_read(m41t93->regmap, M41T93_REG_FLAGS, &tmp);
+	if (ret < 0)
+		return ret;
 
 	if (tmp & M41T93_FLAG_OF) {
 		ret = -EINVAL;
-		dev_warn(&spi->dev, "OF bit is set, write time to restart.\n");
+		dev_warn(dev, "OF bit is set, write time to restart.\n");
 	}
 
 	if (tmp & M41T93_FLAG_BL)
-		dev_warn(&spi->dev, "BL bit is set, replace battery.\n");
+		dev_warn(dev, "BL bit is set, replace battery.\n");
 
 	/* read actual time/date */
-	tmp = spi_write_then_read(spi, &start_addr, 1, buf, sizeof(buf));
-	if (tmp < 0)
-		return tmp;
+	ret = regmap_bulk_read(m41t93->regmap, M41T93_REG_SSEC, buf, sizeof(buf));
+	if (ret < 0)
+		return ret;
 
 	tm->tm_sec	= bcd2bin(buf[M41T93_REG_ST_SEC]);
 	tm->tm_min	= bcd2bin(buf[M41T93_REG_MIN]);
@@ -167,26 +161,51 @@ static const struct rtc_class_ops m41t93_rtc_ops = {
 
 static struct spi_driver m41t93_driver;
 
+static const struct regmap_config regmap_config = {
+	.reg_bits = 8,
+	.val_bits = 8,
+	.read_flag_mask = 0x00,
+	.write_flag_mask = 0x80,
+	.zero_flag_mask = true,
+};
+
 static int m41t93_probe(struct spi_device *spi)
 {
-	struct rtc_device *rtc;
-	int res;
+	int res, ret;
+	struct m41t93_data *m41t93;
 
 	spi->bits_per_word = 8;
 	spi_setup(spi);
 
-	res = spi_w8r8(spi, M41T93_REG_WDAY);
+	m41t93 = devm_kzalloc(&spi->dev, sizeof(struct m41t93_data), GFP_KERNEL);
+
+	if (!m41t93)
+		return -ENOMEM;
+
+	/* Set up regmap to access device registers*/
+	m41t93->regmap = devm_regmap_init_spi(spi, &regmap_config);
+	if (IS_ERR(m41t93->regmap)) {
+		dev_err(&spi->dev, "regmap init failure\n");
+		return PTR_ERR(m41t93->regmap);
+	}
+
+	ret = regmap_read(m41t93->regmap, M41T93_REG_WDAY, &res);
+	if (ret < 0) {
+		dev_err(&spi->dev, "IO error\n");
+		return -EIO;
+	}
+
 	if (res < 0 || (res & 0xf8) != 0) {
 		dev_err(&spi->dev, "not found 0x%x.\n", res);
 		return -ENODEV;
 	}
 
-	rtc = devm_rtc_device_register(&spi->dev, m41t93_driver.driver.name,
-					&m41t93_rtc_ops, THIS_MODULE);
-	if (IS_ERR(rtc))
-		return PTR_ERR(rtc);
+	spi_set_drvdata(spi, m41t93);
 
-	spi_set_drvdata(spi, rtc);
+	m41t93->rtc = devm_rtc_device_register(&spi->dev, m41t93_driver.driver.name,
+					&m41t93_rtc_ops, THIS_MODULE);
+	if (IS_ERR(m41t93->rtc))
+		return PTR_ERR(m41t93->rtc);
 
 	return 0;
 }
-- 
2.34.1

[PATCH 4/7] rtc: m41t93: Add alarm support

[Akhilesh Patil]

Implement alarm feature for rtc-m41t93 by adding necessary
callbacks - set_alarm, read_alarm and alarm_irq_enable.
Enable support to configure alarm 1 out of 2 alarms present in this rtc.

Tested by observing IRQ pin (pin 12 of SOX18 package) on logic analyzer
going low after alarm condition is met.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 101 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index ad862bf706b6..8cc179e08a4a 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -22,6 +22,14 @@
 #define M41T93_REG_DAY			5
 #define M41T93_REG_MON			6
 #define M41T93_REG_YEAR			7
+#define M41T93_REG_AL1_MONTH		0xa
+#define M41T93_REG_AL1_DATE		0xb
+#define M41T93_REG_AL1_HOUR		0xc
+#define M41T93_REG_AL1_MIN		0xd
+#define M41T93_REG_AL1_SEC		0xe
+#define M41T93_BIT_A1IE                 BIT(7)
+#define M41T93_BIT_ABE			BIT(5)
+#define M41T93_FLAG_AF1                 BIT(6)
 
 
 #define M41T93_REG_ALM_HOUR_HT		0xc
@@ -153,10 +161,103 @@ static int m41t93_get_time(struct device *dev, struct rtc_time *tm)
 	return ret;
 }
 
+static int m41t93_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct m41t93_data *m41t93 = dev_get_drvdata(dev);
+	int ret;
+	unsigned int val;
+	u8 alarm_vals[5] = {0};
+
+	ret = regmap_bulk_write(m41t93->regmap, M41T93_REG_AL1_DATE, alarm_vals, 4);
+	if (ret)
+		return ret;
+
+	/* Set alarm values */
+	alarm_vals[0] = bin2bcd(alrm->time.tm_mon + 1) & 0x1f;
+	alarm_vals[1] = bin2bcd(alrm->time.tm_mday) & 0x3f;
+	alarm_vals[2] = bin2bcd(alrm->time.tm_hour) & 0x3f;
+	alarm_vals[3] = bin2bcd(alrm->time.tm_min) & 0x7f;
+	alarm_vals[4] = bin2bcd(alrm->time.tm_sec) & 0x7f;
+
+	if (alrm->enabled) {
+		/* Enable alarm IRQ generation
+		 * Note: SQWE is forced disabled while setting alarm
+		 */
+		alarm_vals[0] |= M41T93_BIT_A1IE | M41T93_BIT_ABE;
+	}
+
+	ret = regmap_bulk_write(m41t93->regmap, M41T93_REG_AL1_MONTH,
+				alarm_vals, sizeof(alarm_vals));
+	if (ret)
+		return ret;
+
+	/* Device address pointer is now at FLAG register, move it to other location
+	 * to finish setting alarm, as recommended by the datasheet.
+	 * We do read of AL1_MONTH register to achieve this.
+	 */
+	ret = regmap_read(m41t93->regmap, M41T93_REG_AL1_MONTH, &val);
+	if (ret)
+		return ret;
+
+	if (bcd2bin(val & 0x1f) == (alrm->time.tm_mon & 0x1f))
+		dev_notice(dev, "Alarm set successfully\n");
+
+	return 0;
+}
+
+static int m41t93_get_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct m41t93_data *m41t93 = dev_get_drvdata(dev);
+	int ret;
+	unsigned int val;
+	u8 alarm_vals[5] = {0};
+
+	ret = regmap_bulk_read(m41t93->regmap, M41T93_REG_AL1_MONTH,
+			       alarm_vals, sizeof(alarm_vals));
+	if (ret)
+		return ret;
+
+	alrm->time.tm_mon = bcd2bin(alarm_vals[0] & 0x1f) - 1;
+	alrm->time.tm_mday = bcd2bin(alarm_vals[1] & 0x3f);
+	alrm->time.tm_hour = bcd2bin(alarm_vals[2] & 0x3f);
+	alrm->time.tm_min = bcd2bin(alarm_vals[3] & 0x7f);
+	alrm->time.tm_sec = bcd2bin(alarm_vals[4] & 0x7f);
+
+	alrm->enabled =  !!(alarm_vals[0] & M41T93_BIT_A1IE);
+
+	ret = regmap_read(m41t93->regmap, M41T93_REG_FLAGS, &val);
+	if (ret)
+		return ret;
+
+	alrm->pending = (val & M41T93_FLAG_AF1) && alrm->enabled;
+
+	return 0;
+}
+
+static int m41t93_alarm_irq_enable(struct device *dev, unsigned int enabled)
+{
+	struct m41t93_data *m41t93 = dev_get_drvdata(dev);
+	unsigned int val;
+	int ret;
+
+	val = enabled ? M41T93_BIT_A1IE | M41T93_BIT_ABE :
+		~(M41T93_BIT_A1IE | M41T93_BIT_ABE);
+
+	ret = regmap_update_bits(m41t93->regmap, M41T93_REG_AL1_MONTH,
+				 M41T93_BIT_A1IE | M41T93_BIT_ABE, val);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
 
 static const struct rtc_class_ops m41t93_rtc_ops = {
 	.read_time	= m41t93_get_time,
 	.set_time	= m41t93_set_time,
+	.set_alarm	= m41t93_set_alarm,
+	.read_alarm	= m41t93_get_alarm,
+	.alarm_irq_enable = m41t93_alarm_irq_enable,
 };
 
 static struct spi_driver m41t93_driver;
-- 
2.34.1

[PATCH 5/7] rtc: m41t93: fix device connection/detection logic during probe

[Akhilesh Patil]

Fix the incorrect assumption about WDAY register (0x4) bits 3 to 7
being 0 after initial power-on to test response from device during probe

Do not expect these bits to be 0 after power on as datasheet does not
explicitly mention these power on defaults but recommends software to
clear these bits during operation. Refer section 3.15 for initial
power-on default bits.

Fix the random probe failures after power on by removing this condition
check. Add alternate response check logic which performs write, read,
compare check on device SRAM register to check device connection.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index 8cc179e08a4a..902797070246 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -30,6 +30,7 @@
 #define M41T93_BIT_A1IE                 BIT(7)
 #define M41T93_BIT_ABE			BIT(5)
 #define M41T93_FLAG_AF1                 BIT(6)
+#define M41T93_SRAM_BASE		0x19
 
 
 #define M41T93_REG_ALM_HOUR_HT		0xc
@@ -290,17 +291,25 @@ static int m41t93_probe(struct spi_device *spi)
 		return PTR_ERR(m41t93->regmap);
 	}
 
-	ret = regmap_read(m41t93->regmap, M41T93_REG_WDAY, &res);
-	if (ret < 0) {
+	ret = regmap_write(m41t93->regmap, M41T93_SRAM_BASE, 0xA5);
+	if (ret) {
 		dev_err(&spi->dev, "IO error\n");
 		return -EIO;
 	}
 
-	if (res < 0 || (res & 0xf8) != 0) {
-		dev_err(&spi->dev, "not found 0x%x.\n", res);
+	ret = regmap_read(m41t93->regmap, M41T93_SRAM_BASE, &res);
+	if (ret) {
+		dev_err(&spi->dev, "IO error\n");
+		return -EIO;
+	}
+
+	if (res != 0xA5) {
+		dev_err(&spi->dev, "No valid response from device 0x%x.\n", res);
 		return -ENODEV;
 	}
 
+	dev_notice(&spi->dev, "m41t93 device response success\n");
+
 	spi_set_drvdata(spi, m41t93);
 
 	m41t93->rtc = devm_rtc_device_register(&spi->dev, m41t93_driver.driver.name,
-- 
2.34.1

Re: [PATCH 5/7] rtc: m41t93: fix device connection/detection logic during probe

[Alexandre Belloni]

On 03/09/2025 19:57:21+0530, Akhilesh Patil wrote:
> Fix the incorrect assumption about WDAY register (0x4) bits 3 to 7
> being 0 after initial power-on to test response from device during probe
> 
> Do not expect these bits to be 0 after power on as datasheet does not
> explicitly mention these power on defaults but recommends software to
> clear these bits during operation. Refer section 3.15 for initial
> power-on default bits.
> 
> Fix the random probe failures after power on by removing this condition
> check. Add alternate response check logic which performs write, read,
> compare check on device SRAM register to check device connection.
> 
> Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
> ---
>  drivers/rtc/rtc-m41t93.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
> index 8cc179e08a4a..902797070246 100644
> --- a/drivers/rtc/rtc-m41t93.c
> +++ b/drivers/rtc/rtc-m41t93.c
> @@ -30,6 +30,7 @@
>  #define M41T93_BIT_A1IE                 BIT(7)
>  #define M41T93_BIT_ABE			BIT(5)
>  #define M41T93_FLAG_AF1                 BIT(6)
> +#define M41T93_SRAM_BASE		0x19
>  
>  
>  #define M41T93_REG_ALM_HOUR_HT		0xc
> @@ -290,17 +291,25 @@ static int m41t93_probe(struct spi_device *spi)
>  		return PTR_ERR(m41t93->regmap);
>  	}
>  
> -	ret = regmap_read(m41t93->regmap, M41T93_REG_WDAY, &res);
> -	if (ret < 0) {
> +	ret = regmap_write(m41t93->regmap, M41T93_SRAM_BASE, 0xA5);

Nope, probe is not called at RTC power on but when linux starts. The
whole point of the RTC is to survive Linux. Writing to this register is
breaking functionnality.

> +	if (ret) {
>  		dev_err(&spi->dev, "IO error\n");
>  		return -EIO;
>  	}
>  
> -	if (res < 0 || (res & 0xf8) != 0) {
> -		dev_err(&spi->dev, "not found 0x%x.\n", res);
> +	ret = regmap_read(m41t93->regmap, M41T93_SRAM_BASE, &res);
> +	if (ret) {
> +		dev_err(&spi->dev, "IO error\n");
> +		return -EIO;
> +	}
> +
> +	if (res != 0xA5) {
> +		dev_err(&spi->dev, "No valid response from device 0x%x.\n", res);
>  		return -ENODEV;
>  	}
>  
> +	dev_notice(&spi->dev, "m41t93 device response success\n");
> +

This is too verbose.

>  	spi_set_drvdata(spi, m41t93);
>  
>  	m41t93->rtc = devm_rtc_device_register(&spi->dev, m41t93_driver.driver.name,
> -- 
> 2.34.1
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

[PATCH 6/7] rtc: m41t93: Add square wave clock provider support

[Akhilesh Patil]

Implement support to configure square wave output (SQW) of m41t93 rtc
via common clock framework clock provider api. Add clock provider
callbacks to control output frequency ranging from 1Hz to 32KHz as
supported by this rtc chip.

Tested by measuring various frequencies on pull-up connected SWQ(7) pin
of m41t93 rtc chip using logic analyzer.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 154 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 154 insertions(+)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index 902797070246..83cc34c4baae 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -13,6 +13,7 @@
 #include <linux/rtc.h>
 #include <linux/spi/spi.h>
 #include <linux/regmap.h>
+#include <linux/clk-provider.h>
 
 #define M41T93_REG_SSEC			0
 #define M41T93_REG_ST_SEC		1
@@ -31,6 +32,10 @@
 #define M41T93_BIT_ABE			BIT(5)
 #define M41T93_FLAG_AF1                 BIT(6)
 #define M41T93_SRAM_BASE		0x19
+#define M41T93_REG_SQW			0x13
+#define M41T93_SQW_RS_MASK		0xf0
+#define M41T93_SQW_RS_SHIFT		4
+#define M41T93_BIT_SQWE			BIT(6)
 
 
 #define M41T93_REG_ALM_HOUR_HT		0xc
@@ -44,6 +49,9 @@
 struct m41t93_data {
 	struct rtc_device *rtc;
 	struct regmap *regmap;
+#ifdef CONFIG_COMMON_CLK
+	struct clk_hw clks;
+#endif
 };
 
 static int m41t93_set_time(struct device *dev, struct rtc_time *tm)
@@ -261,6 +269,146 @@ static const struct rtc_class_ops m41t93_rtc_ops = {
 	.alarm_irq_enable = m41t93_alarm_irq_enable,
 };
 
+#ifdef CONFIG_COMMON_CLK
+#define clk_sqw_to_m41t93_data(clk)	\
+	container_of(clk, struct m41t93_data, clks)
+
+/* m41t93 RTC clock output support */
+static unsigned long m41t93_clk_rates[] = {
+	0,
+	32768, /* RS3:RS0 = 0b0001 */
+	8192,
+	4096,
+	2048,
+	1024,
+	512,
+	256,
+	128,
+	64,
+	32,
+	16,
+	8,
+	4,
+	2,
+	1, /* RS3:RS0 = 0b1111 */
+};
+
+static unsigned long m41t93_clk_sqw_recalc_rate(struct clk_hw *hw,
+						  unsigned long parent_rate)
+{
+	int ret;
+	unsigned int rate_id;
+	struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
+
+	ret = regmap_read(m41t93->regmap, M41T93_REG_SQW, &rate_id);
+	if (ret)
+		return ret;
+
+	rate_id &= M41T93_SQW_RS_MASK;
+	rate_id >>= M41T93_SQW_RS_SHIFT;
+
+	return m41t93_clk_rates[rate_id];
+}
+
+static int m41t93_clk_sqw_determine_rate(struct clk_hw *hw,
+					 struct clk_rate_request *req)
+{
+	int i;
+
+	for (i = 1; i < ARRAY_SIZE(m41t93_clk_rates); i++) {
+		if (req->rate >= m41t93_clk_rates[i]) {
+			req->rate = m41t93_clk_rates[i];
+			return 0;
+		}
+	}
+
+	return 0;
+}
+
+static int m41t93_clk_sqw_set_rate(struct clk_hw *hw, unsigned long rate,
+				   unsigned long parent_rate)
+{
+	int id, ret;
+	struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
+
+	for (id = 0; id < ARRAY_SIZE(m41t93_clk_rates); id++) {
+		if (m41t93_clk_rates[id] == rate)
+			break;
+	}
+
+	if (id >= ARRAY_SIZE(m41t93_clk_rates))
+		return -EINVAL;
+
+	ret = regmap_update_bits(m41t93->regmap, M41T93_REG_SQW,
+				 M41T93_SQW_RS_MASK, id << M41T93_SQW_RS_SHIFT);
+
+	return ret;
+}
+
+static int m41t93_clk_sqw_prepare(struct clk_hw *hw)
+{
+	int ret;
+	struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
+
+	ret = regmap_update_bits(m41t93->regmap, M41T93_REG_AL1_MONTH,
+				 M41T93_BIT_SQWE, M41T93_BIT_SQWE);
+
+	return ret;
+}
+
+static void m41t93_clk_sqw_unprepare(struct clk_hw *hw)
+{
+	struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
+
+	regmap_update_bits(m41t93->regmap, M41T93_REG_AL1_MONTH,
+			   M41T93_BIT_SQWE, ~M41T93_BIT_SQWE);
+}
+
+static int m41t93_clk_sqw_is_prepared(struct clk_hw *hw)
+{
+	int ret;
+	struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
+	unsigned int status;
+
+	ret = regmap_read(m41t93->regmap, M41T93_REG_AL1_MONTH, &status);
+	if (ret)
+		return ret;
+
+	return !!(status & M41T93_BIT_SQWE);
+}
+
+static const struct clk_ops m41t93_clk_sqw_ops = {
+	.prepare = m41t93_clk_sqw_prepare,
+	.unprepare = m41t93_clk_sqw_unprepare,
+	.is_prepared = m41t93_clk_sqw_is_prepared,
+	.recalc_rate = m41t93_clk_sqw_recalc_rate,
+	.set_rate = m41t93_clk_sqw_set_rate,
+	.determine_rate = m41t93_clk_sqw_determine_rate,
+};
+
+static int rtc_m41t93_clks_register(struct device *dev, struct m41t93_data *m41t93)
+{
+	struct device_node *node = dev->of_node;
+	struct clk *clk;
+	struct clk_init_data init = {0};
+
+	init.name = "m41t93_clk_sqw";
+	init.ops = &m41t93_clk_sqw_ops;
+
+	m41t93->clks.init = &init;
+
+	/* Register the clock with CCF */
+	clk = devm_clk_register(dev, &m41t93->clks);
+	if (IS_ERR(clk))
+		return PTR_ERR(clk);
+
+	if (node)
+		of_clk_add_provider(node, of_clk_src_simple_get, clk);
+
+	return 0;
+}
+#endif
+
 static struct spi_driver m41t93_driver;
 
 static const struct regmap_config regmap_config = {
@@ -317,6 +465,12 @@ static int m41t93_probe(struct spi_device *spi)
 	if (IS_ERR(m41t93->rtc))
 		return PTR_ERR(m41t93->rtc);
 
+#ifdef CONFIG_COMMON_CLK
+	ret = rtc_m41t93_clks_register(&spi->dev, m41t93);
+	if (ret)
+		dev_warn(&spi->dev, "Unable to register clock\n");
+#endif
+
 	return 0;
 }
 
-- 
2.34.1

[PATCH 7/7] rtc: m41t93: Add watchdog support

[Akhilesh Patil]

Implement watchdog feature driver for m41t93 rtc with 1s resolution and
alarm only mode. Define start, stop, ping, and set_timeout callbacks
as needed by watchdog framework.

Tested by observing IRQ pin(12) going low after intentionally not pinging
watchdog.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
---
 drivers/rtc/rtc-m41t93.c | 93 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)

diff --git a/drivers/rtc/rtc-m41t93.c b/drivers/rtc/rtc-m41t93.c
index 83cc34c4baae..e549c5f1e11a 100644
--- a/drivers/rtc/rtc-m41t93.c
+++ b/drivers/rtc/rtc-m41t93.c
@@ -14,6 +14,7 @@
 #include <linux/spi/spi.h>
 #include <linux/regmap.h>
 #include <linux/clk-provider.h>
+#include <linux/watchdog.h>
 
 #define M41T93_REG_SSEC			0
 #define M41T93_REG_ST_SEC		1
@@ -36,6 +37,10 @@
 #define M41T93_SQW_RS_MASK		0xf0
 #define M41T93_SQW_RS_SHIFT		4
 #define M41T93_BIT_SQWE			BIT(6)
+#define M41T93_REG_WATCHDOG		0x9
+#define M41T93_WDT_RB_MASK		0x3
+#define M41T93_WDT_BMB_MASK		0x7c
+#define M41T93_WDT_BMB_SHIFT		2
 
 
 #define M41T93_REG_ALM_HOUR_HT		0xc
@@ -52,6 +57,7 @@ struct m41t93_data {
 #ifdef CONFIG_COMMON_CLK
 	struct clk_hw clks;
 #endif
+	struct watchdog_device wdd;
 };
 
 static int m41t93_set_time(struct device *dev, struct rtc_time *tm)
@@ -409,6 +415,90 @@ static int rtc_m41t93_clks_register(struct device *dev, struct m41t93_data *m41t
 }
 #endif
 
+static int m41t93_wdt_ping(struct watchdog_device *wdd)
+{
+	u8 resolution, mult;
+	u8 val = 0;
+	int ret;
+	struct m41t93_data *m41t93 = watchdog_get_drvdata(wdd);
+
+	/*  Resolution supported by hardware
+	 *  0b00 : 1/16 seconds
+	 *  0b01 : 1/4 second
+	 *  0b10 : 1 second
+	 *  0b11 : 4 seconds
+	 */
+	resolution = 0x2; /* hardcode resolution to 1s */
+	mult = wdd->timeout;
+	val = resolution | (mult << M41T93_WDT_BMB_SHIFT &  M41T93_WDT_BMB_MASK);
+
+	ret = regmap_write_bits(m41t93->regmap, M41T93_REG_WATCHDOG,
+				 M41T93_WDT_RB_MASK | M41T93_WDT_BMB_MASK, val);
+
+	return ret;
+}
+
+static int m41t93_wdt_start(struct watchdog_device *wdd)
+{
+	return m41t93_wdt_ping(wdd);
+}
+
+static int m41t93_wdt_stop(struct watchdog_device *wdd)
+{
+	struct m41t93_data *m41t93 = watchdog_get_drvdata(wdd);
+
+	/* Write 0 to watchdog register */
+	return regmap_write_bits(m41t93->regmap, M41T93_REG_WATCHDOG,
+				  M41T93_WDT_RB_MASK | M41T93_WDT_BMB_MASK, 0);
+}
+
+static int m41t93_wdt_set_timeout(struct watchdog_device *wdd,
+				   unsigned int new_timeout)
+{
+	wdd->timeout = new_timeout;
+
+	return 0;
+}
+
+static const struct watchdog_info m41t93_wdt_info = {
+	.identity = "m41t93 rtc Watchdog",
+	.options = WDIOF_ALARMONLY,
+};
+
+static const struct watchdog_ops m41t93_watchdog_ops = {
+	.owner = THIS_MODULE,
+	.start = m41t93_wdt_start,
+	.stop = m41t93_wdt_stop,
+	.ping = m41t93_wdt_ping,
+	.set_timeout = m41t93_wdt_set_timeout,
+};
+
+static int m41t93_watchdog_register(struct device *dev, struct m41t93_data *m41t93)
+{
+	int ret;
+
+	m41t93->wdd.parent = dev;
+	m41t93->wdd.info = &m41t93_wdt_info;
+	m41t93->wdd.ops = &m41t93_watchdog_ops;
+	m41t93->wdd.min_timeout = 0;
+	m41t93->wdd.max_timeout = 10;
+	m41t93->wdd.timeout = 3; /* Default timeout is 3 sec */
+	m41t93->wdd.status = WATCHDOG_NOWAYOUT_INIT_STATUS;
+
+	watchdog_set_drvdata(&m41t93->wdd, m41t93);
+
+	ret = devm_watchdog_register_device(dev, &m41t93->wdd);
+	if (ret) {
+		dev_warn(dev, "Failed to register watchdog\n");
+		return ret;
+	}
+
+	/* Disable watchdog at start */
+	ret = m41t93_wdt_stop(&m41t93->wdd);
+
+	return ret;
+}
+
 static struct spi_driver m41t93_driver;
 
 static const struct regmap_config regmap_config = {
@@ -470,6 +560,9 @@ static int m41t93_probe(struct spi_device *spi)
 	if (ret)
 		dev_warn(&spi->dev, "Unable to register clock\n");
 #endif
+	ret = m41t93_watchdog_register(&spi->dev, m41t93);
+	if (ret)
+		dev_warn(&spi->dev, "Unable to register watchdog\n");
 
 	return 0;
 }
-- 
2.34.1