[PATCH v3 0/3] hwmon: (ads7871) Modernize and fix DMA safety

[PATCH v3 0/3] hwmon: (ads7871) Modernize and fix DMA safety

[Tabrez Ahmed]

This series modernizes the ads7871 driver by migrating it to the
hwmon_device_register_with_info() API and moving the SPI transfer
buffer into the driver's private data structure to ensure it is
DMA-safe.

Changes in v3:
- Added Patch 1 to fix a pre-existing Endianness bug in the 16-bit
  register read logic, flagged by Sashiko AI during v2 review.
- Fixed multiple formatting and alignment issues caught by checkpatch
  --strict, as requested by Guenter.
- Added "While at it, fix checkpatch violations" to Patch 2 commit message.

Changes in v2:
- Dropped custom mutex in favor of native hwmon core serialization.
- Split API migration and DMA fix into separate, logical patches.
- Corrected output scaling and sign extension to meet hwmon ABI.

Tabrez Ahmed (3):
  hwmon: (ads7871) Fix endianness bug in 16-bit register reads
  hwmon: (ads7871) Convert to hwmon_device_register_with_info
  hwmon: (ads7871) Use DMA-safe buffer for SPI writes
Tabrez Ahmed (3):
  hwmon: (ads7871) Fix endianness bug in 16-bit register reads
  hwmon: (ads7871) Convert to hwmon_device_register_with_info
  hwmon: (ads7871) Use DMA-safe buffer for SPI writes

 drivers/hwmon/ads7871.c | 118 +++++++++++++++++++++++-----------------
 1 file changed, 68 insertions(+), 50 deletions(-)

-- 
2.43.0

[PATCH v3 1/3] hwmon: (ads7871) Fix endianness bug in 16-bit register reads

[Tabrez Ahmed]

The ads7871_read_reg16() function relies on spi_w8r16() to read the
16-bit sensor output. The ADS7871 device transmits the Least Significant
Byte (LSB) first.

On Little-Endian architectures, spi_w8r16() correctly reconstructs the
16-bit value. However, on Big-Endian architectures, the byte swapping
causes the first received byte (LSB) to be placed in the most significant
byte of the u16, resulting in corrupted voltage readings.

Replace spi_w8r16() with a manual spi_write_then_read() into a byte array,
and safely reconstruct the integer using get_unaligned_le16() to ensure
correct behavior across all architectures.

Reported-by: Sashiko <sashiko@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260329073352.270451-1-tabreztalks%40gmail.com
Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
---
 drivers/hwmon/ads7871.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
index 9bfdf9e6bcd77..9b52aa496d522 100644
--- a/drivers/hwmon/ads7871.c
+++ b/drivers/hwmon/ads7871.c
@@ -59,6 +59,7 @@
 #include <linux/hwmon-sysfs.h>
 #include <linux/err.h>
 #include <linux/delay.h>
+#include <asm/unaligned.h>
 
 #define DEVICE_NAME	"ads7871"
 
@@ -77,9 +78,14 @@ static int ads7871_read_reg8(struct spi_device *spi, int reg)
 static int ads7871_read_reg16(struct spi_device *spi, int reg)
 {
 	int ret;
+	u8 buf[2];
+
 	reg = reg | INST_READ_BM | INST_16BIT_BM;
-	ret = spi_w8r16(spi, reg);
-	return ret;
+	ret = spi_write_then_read(spi, &reg, 1, buf, 2);
+	if (ret < 0)
+		return ret;
+
+	return get_unaligned_le16(buf);
 }
 
 static int ads7871_write_reg8(struct spi_device *spi, int reg, u8 val)
-- 
2.43.0

[PATCH v3 2/3] hwmon: (ads7871) Convert to hwmon_device_register_with_info

[Tabrez Ahmed]

Convert the ads7871 driver from the legacy hwmon_device_register() to the
modern hwmon_device_register_with_info() API. This migration simplifies
the driver by using the structured hwmon_channel_info approach and
prepares the codebase for the transition to a shared DMA-safe buffer.
While at it, fix checkpatch violations.

Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
---
 drivers/hwmon/ads7871.c | 75 ++++++++++++++++++++++-------------------
 1 file changed, 40 insertions(+), 35 deletions(-)

diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
index 9b52aa496d522..851b70e9f6ba2 100644
--- a/drivers/hwmon/ads7871.c
+++ b/drivers/hwmon/ads7871.c
@@ -56,7 +56,6 @@
 #include <linux/init.h>
 #include <linux/spi/spi.h>
 #include <linux/hwmon.h>
-#include <linux/hwmon-sysfs.h>
 #include <linux/err.h>
 #include <linux/delay.h>
 #include <asm/unaligned.h>
@@ -67,6 +66,16 @@ struct ads7871_data {
 	struct spi_device *spi;
 };
 
+static umode_t ads7871_is_visible(const void *data,
+				  enum hwmon_sensor_types type,
+				  u32 attr, int channel)
+{
+	if (type == hwmon_in && attr == hwmon_in_input)
+		return 0444;
+
+	return 0;
+}
+
 static int ads7871_read_reg8(struct spi_device *spi, int reg)
 {
 	int ret;
@@ -91,19 +100,20 @@ static int ads7871_read_reg16(struct spi_device *spi, int reg)
 static int ads7871_write_reg8(struct spi_device *spi, int reg, u8 val)
 {
 	u8 tmp[2] = {reg, val};
+
 	return spi_write(spi, tmp, sizeof(tmp));
 }
 
-static ssize_t voltage_show(struct device *dev, struct device_attribute *da,
-			    char *buf)
+static int ads7871_read(struct device *dev, enum hwmon_sensor_types type,
+			u32 attr, int channel, long *val)
 {
 	struct ads7871_data *pdata = dev_get_drvdata(dev);
 	struct spi_device *spi = pdata->spi;
-	struct sensor_device_attribute *attr = to_sensor_dev_attr(da);
-	int ret, val, i = 0;
-	uint8_t channel, mux_cnv;
+	int ret, raw_val, i = 0;
+	u8 mux_cnv;
 
-	channel = attr->index;
+	if (type != hwmon_in || attr != hwmon_in_input)
+		return -EOPNOTSUPP;
 	/*
 	 * TODO: add support for conversions
 	 * other than single ended with a gain of 1
@@ -133,39 +143,34 @@ static ssize_t voltage_show(struct device *dev, struct device_attribute *da,
 	}
 
 	if (mux_cnv == 0) {
-		val = ads7871_read_reg16(spi, REG_LS_BYTE);
-		if (val < 0)
-			return val;
+		raw_val = ads7871_read_reg16(spi, REG_LS_BYTE);
+		if (raw_val < 0)
+			return raw_val;
+
 		/*result in volts*10000 = (val/8192)*2.5*10000*/
-		val = ((val >> 2) * 25000) / 8192;
-		return sysfs_emit(buf, "%d\n", val);
+		*val = ((raw_val >> 2) * 25000) / 8192;
+		return 0;
 	}
 
 	return -ETIMEDOUT;
 }
 
-static SENSOR_DEVICE_ATTR_RO(in0_input, voltage, 0);
-static SENSOR_DEVICE_ATTR_RO(in1_input, voltage, 1);
-static SENSOR_DEVICE_ATTR_RO(in2_input, voltage, 2);
-static SENSOR_DEVICE_ATTR_RO(in3_input, voltage, 3);
-static SENSOR_DEVICE_ATTR_RO(in4_input, voltage, 4);
-static SENSOR_DEVICE_ATTR_RO(in5_input, voltage, 5);
-static SENSOR_DEVICE_ATTR_RO(in6_input, voltage, 6);
-static SENSOR_DEVICE_ATTR_RO(in7_input, voltage, 7);
-
-static struct attribute *ads7871_attrs[] = {
-	&sensor_dev_attr_in0_input.dev_attr.attr,
-	&sensor_dev_attr_in1_input.dev_attr.attr,
-	&sensor_dev_attr_in2_input.dev_attr.attr,
-	&sensor_dev_attr_in3_input.dev_attr.attr,
-	&sensor_dev_attr_in4_input.dev_attr.attr,
-	&sensor_dev_attr_in5_input.dev_attr.attr,
-	&sensor_dev_attr_in6_input.dev_attr.attr,
-	&sensor_dev_attr_in7_input.dev_attr.attr,
+static const struct hwmon_channel_info * const ads7871_info[] = {
+	HWMON_CHANNEL_INFO(in,
+			   HWMON_I_INPUT, HWMON_I_INPUT, HWMON_I_INPUT, HWMON_I_INPUT,
+			   HWMON_I_INPUT, HWMON_I_INPUT, HWMON_I_INPUT, HWMON_I_INPUT),
 	NULL
 };
 
-ATTRIBUTE_GROUPS(ads7871);
+static const struct hwmon_ops ads7871_hwmon_ops = {
+	.is_visible = ads7871_is_visible,
+	.read = ads7871_read,
+};
+
+static const struct hwmon_chip_info ads7871_chip_info = {
+	.ops = &ads7871_hwmon_ops,
+	.info = ads7871_info,
+};
 
 static int ads7871_probe(struct spi_device *spi)
 {
@@ -200,10 +205,10 @@ static int ads7871_probe(struct spi_device *spi)
 		return -ENOMEM;
 
 	pdata->spi = spi;
-
-	hwmon_dev = devm_hwmon_device_register_with_groups(dev, spi->modalias,
-							   pdata,
-							   ads7871_groups);
+	hwmon_dev = devm_hwmon_device_register_with_info(dev, spi->modalias,
+							 pdata,
+							 &ads7871_chip_info,
+							 NULL);
 	return PTR_ERR_OR_ZERO(hwmon_dev);
 }
 
-- 
2.43.0

[PATCH v3 3/3] hwmon: (ads7871) Use DMA-safe buffer for SPI writes

[Tabrez Ahmed]

The driver currently passes a stack-allocated buffer to spi_write(),
which is incompatible with DMA on systems with CONFIG_VMAP_STACK
enabled.

Move the transfer buffer into the driver's private data structure
to ensure it is DMA-safe. Since this shared buffer now requires
serialization, this change depends on the previous commit which
migrated the driver to the hwmon 'with_info' API.

While moving the logic, also:
- Corrected the sign extension for 14-bit data by casting to s16.
- Scaled the output to millivolts (2500mV full scale
) to comply with the hwmon ABI.

Signed-off-by: Tabrez Ahmed <tabreztalks@gmail.com>
---
 drivers/hwmon/ads7871.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
index 851b70e9f6ba2..d0f1eec8effbf 100644
--- a/drivers/hwmon/ads7871.c
+++ b/drivers/hwmon/ads7871.c
@@ -64,6 +64,7 @@
 
 struct ads7871_data {
 	struct spi_device *spi;
+	u8 tx_buf[2] ____cacheline_aligned;
 };
 
 static umode_t ads7871_is_visible(const void *data,
@@ -97,11 +98,12 @@ static int ads7871_read_reg16(struct spi_device *spi, int reg)
 	return get_unaligned_le16(buf);
 }
 
-static int ads7871_write_reg8(struct spi_device *spi, int reg, u8 val)
+static int ads7871_write_reg8(struct ads7871_data *pdata, int reg, u8 val)
 {
-	u8 tmp[2] = {reg, val};
+	pdata->tx_buf[0] = reg;
+	pdata->tx_buf[1] = val;
 
-	return spi_write(spi, tmp, sizeof(tmp));
+	return spi_write(pdata->spi, pdata->tx_buf, 2);
 }
 
 static int ads7871_read(struct device *dev, enum hwmon_sensor_types type,
@@ -120,7 +122,7 @@ static int ads7871_read(struct device *dev, enum hwmon_sensor_types type,
 	 */
 	/*MUX_M3_BM forces single ended*/
 	/*This is also where the gain of the PGA would be set*/
-	ret = ads7871_write_reg8(spi, REG_GAIN_MUX,
+	ret = ads7871_write_reg8(pdata, REG_GAIN_MUX,
 				 (MUX_CNV_BM | MUX_M3_BM | channel));
 	if (ret < 0)
 		return ret;
@@ -128,6 +130,7 @@ static int ads7871_read(struct device *dev, enum hwmon_sensor_types type,
 	ret = ads7871_read_reg8(spi, REG_GAIN_MUX);
 	if (ret < 0)
 		return ret;
+
 	mux_cnv = ((ret & MUX_CNV_BM) >> MUX_CNV_BV);
 	/*
 	 * on 400MHz arm9 platform the conversion
@@ -147,8 +150,11 @@ static int ads7871_read(struct device *dev, enum hwmon_sensor_types type,
 		if (raw_val < 0)
 			return raw_val;
 
-		/*result in volts*10000 = (val/8192)*2.5*10000*/
-		*val = ((raw_val >> 2) * 25000) / 8192;
+		/*
+		 * Use (s16) to ensure the sign bit is preserved during the shift.
+		 * Report millivolts (2.5V = 2500mV).
+		 */
+		*val = ((s16)raw_val >> 2) * 2500 / 8192;
 		return 0;
 	}
 
@@ -185,11 +191,17 @@ static int ads7871_probe(struct spi_device *spi)
 	spi->bits_per_word = 8;
 	spi_setup(spi);
 
-	ads7871_write_reg8(spi, REG_SER_CONTROL, 0);
-	ads7871_write_reg8(spi, REG_AD_CONTROL, 0);
+	pdata = devm_kzalloc(dev, sizeof(*pdata), GFP_KERNEL);
+	if (!pdata)
+		return -ENOMEM;
+
+	pdata->spi = spi;
+
+	ads7871_write_reg8(pdata, REG_SER_CONTROL, 0);
+	ads7871_write_reg8(pdata, REG_AD_CONTROL, 0);
 
 	val = (OSC_OSCR_BM | OSC_OSCE_BM | OSC_REFE_BM | OSC_BUFE_BM);
-	ads7871_write_reg8(spi, REG_OSC_CONTROL, val);
+	ads7871_write_reg8(pdata, REG_OSC_CONTROL, val);
 	ret = ads7871_read_reg8(spi, REG_OSC_CONTROL);
 
 	dev_dbg(dev, "REG_OSC_CONTROL write:%x, read:%x\n", val, ret);
@@ -200,11 +212,6 @@ static int ads7871_probe(struct spi_device *spi)
 	if (val != ret)
 		return -ENODEV;
 
-	pdata = devm_kzalloc(dev, sizeof(struct ads7871_data), GFP_KERNEL);
-	if (!pdata)
-		return -ENOMEM;
-
-	pdata->spi = spi;
 	hwmon_dev = devm_hwmon_device_register_with_info(dev, spi->modalias,
 							 pdata,
 							 &ads7871_chip_info,
-- 
2.43.0