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

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Message-ID: <c3deec9e679cd4e4a49a2cc1cba340c552faefdc.1756908788.git.akhilesh@ee.iitb.ac.in>
Date: Wed, 3 Sep 2025 19:57:21 +0530
From: Akhilesh Patil <akhilesh@...iitb.ac.in>
To: alexandre.belloni@...tlin.com, krzk+dt@...nel.org, robh@...nel.org,
	conor+dt@...nel.org
Cc: skhan@...uxfoundation.org, linux-rtc@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	akhileshpatilvnit@...il.com
Subject: [PATCH 5/7] rtc: m41t93: fix device connection/detection logic
 during probe

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@...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