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

Hi Akhilesh,

kernel test robot noticed the following build warnings:

[auto build test WARNING on abelloni/rtc-next]
[also build test WARNING on linus/master v6.17-rc4 next-20250903]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Akhilesh-Patil/rtc-m41t93-add-device-tree-support/20250903-223155
base:   https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/linux.git rtc-next
patch link:    https://lore.kernel.org/r/c53cdf7c2af95160e05cb4db343bb172a88ae7c9.1756908788.git.akhilesh%40ee.iitb.ac.in
patch subject: [PATCH 6/7] rtc: m41t93: Add square wave clock provider support
config: x86_64-buildonly-randconfig-001-20250904 (https://download.01.org/0day-ci/archive/20250904/202509041224.AOsBArcW-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.4.0-5) 12.4.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250904/202509041224.AOsBArcW-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202509041224.AOsBArcW-lkp@intel.com/

All warnings (new ones prefixed by >>):

   drivers/rtc/rtc-m41t93.c: In function 'm41t93_clk_sqw_unprepare':
>> drivers/rtc/rtc-m41t93.c:364:45: warning: conversion from 'long unsigned int' to 'unsigned int' changes value from '18446744073709551551' to '4294967231' [-Woverflow]
     364 |                            M41T93_BIT_SQWE, ~M41T93_BIT_SQWE);


vim +364 drivers/rtc/rtc-m41t93.c

   358	
   359	static void m41t93_clk_sqw_unprepare(struct clk_hw *hw)
   360	{
   361		struct m41t93_data *m41t93 = clk_sqw_to_m41t93_data(hw);
   362	
   363		regmap_update_bits(m41t93->regmap, M41T93_REG_AL1_MONTH,
 > 364				   M41T93_BIT_SQWE, ~M41T93_BIT_SQWE);
   365	}
   366	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki