[PATCH] hwmon: (lm75) Fix setting polarity mask for chips with 16-bit config register

[PATCH] hwmon: (lm75) Fix setting polarity mask for chips with 16-bit config register

[Guenter Roeck]

When probing a 16-bit compatible temperature sensor configured with the
"ti,alert-polarity-active-high" device property, lm75_generic_probe()
writes to a reserved bit instead of the alert polarity bit due to an
incorrect bit shift in the 16-bit polarity mask.
On 16-bit sensors, the polarity bit is physically located at bit 2 of the
MSByte. When accessed via i2c_smbus_read_word_data() or
i2c_smbus_write_word_data(), the SMBus protocol maps the first received
byte (the physical MSByte) to the lower 8 bits of the integer
representation. Consequently, the polarity bit is consistently mapped to
bit 2, identically to 8-bit sensors. However, the
LM75_ALERT_POLARITY_HIGH_16_BIT macro applies an 8-bit left shift,
erroneously targeting bit 10. This causes the driver to silently corrupt a
reserved bit in the configuration register while leaving the active-high
alert polarity unconfigured, breaking the polarity feature.

Use BIT(2) as polarity mask in the configuration register for all chips
to fix the problem.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: 1be4001d9b9b ("hwmon: (lm75) Support active-high alert polarity")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/hwmon/lm75.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
index 2d2d752aeac9..917ac250a648 100644
--- a/drivers/hwmon/lm75.c
+++ b/drivers/hwmon/lm75.c
@@ -123,8 +123,7 @@ struct lm75_data {
 
 static const u8 lm75_sample_set_masks[] = { 0 << 5, 1 << 5, 2 << 5, 3 << 5 };
 
-#define LM75_ALERT_POLARITY_HIGH_8_BIT	(BIT(2))
-#define LM75_ALERT_POLARITY_HIGH_16_BIT	(BIT(2) << 8)
+#define LM75_ALERT_POLARITY_HIGH_BIT	BIT(2)
 #define LM75_SAMPLE_CLEAR_MASK		(3 << 5)
 
 /* The structure below stores the configuration values of the supported devices.
@@ -770,11 +769,7 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 	data->orig_conf = status;
 
 	/* Enforce polarity active-low (default) or active-high (devicetree) */
-	if (!data->params->config_reg_16bits)
-		pol_mask = LM75_ALERT_POLARITY_HIGH_8_BIT;
-	else
-		pol_mask = LM75_ALERT_POLARITY_HIGH_16_BIT;
-
+	pol_mask = LM75_ALERT_POLARITY_HIGH_BIT;
 	clr_mask = data->params->clr_mask | pol_mask;
 	set_mask = data->params->set_mask & ~pol_mask;
 	if (device_property_read_bool(dev, "ti,alert-polarity-active-high"))
-- 
2.45.2

Re: [PATCH] hwmon: (lm75) Fix setting polarity mask for chips with 16-bit config register

[sashiko-bot]

> When probing a 16-bit compatible temperature sensor configured with the
> "ti,alert-polarity-active-high" device property, lm75_generic_probe()
> writes to a reserved bit instead of the alert polarity bit due to an
> incorrect bit shift in the 16-bit polarity mask.
> On 16-bit sensors, the polarity bit is physically located at bit 2 of the
> MSByte. When accessed via i2c_smbus_read_word_data() or
> i2c_smbus_write_word_data(), the SMBus protocol maps the first received
> byte (the physical MSByte) to the lower 8 bits of the integer
> representation. Consequently, the polarity bit is consistently mapped to
> bit 2, identically to 8-bit sensors. However, the
> LM75_ALERT_POLARITY_HIGH_16_BIT macro applies an 8-bit left shift,
> erroneously targeting bit 10. This causes the driver to silently corrupt a
> reserved bit in the configuration register while leaving the active-high
> alert polarity unconfigured, breaking the polarity feature.
> 
> Use BIT(2) as polarity mask in the configuration register for all chips
> to fix the problem.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Fixes: 1be4001d9b9b ("hwmon: (lm75) Support active-high alert polarity")
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261031034755.1079779-1-linux@roeck-us.net?part=1