[PATCH] hwmon: (lm87) Propagate analog output write errors

[pocj8ur4in <pocj8ur4in@gmail.com>]

From: "Baek Kyu, Kim" <pocj8ur4in@gmail.com>

The aout_output store path updates its cached value before writing the
register and ignores the SMBus result. A rejected write therefore returns
a successful byte count and reports the requested value until the cache
expires. With a cached 980 mV, a rejected write of 1960 mV returned 5 and
read back as 1960 mV although the register still held 980 mV.

Check the write result and update the cache only after success. Return the
bus error to the sysfs caller.

Signed-off-by: Baek Kyu, Kim <pocj8ur4in@gmail.com>
---
Tested on ARM64 QEMU/KVM through the lm87 and adm1024 sysfs paths,
using an emulated SMBus adapter. With 980 mV cached, a pre-apply
-EIO rejection of a 1960 mV write returned 5 and read back 1960 mV
before the change; after the change, it returned -EIO and read back
980 mV. The same behavior was checked with -EREMOTEIO. No physical
chip or analog voltage was measured.

 drivers/hwmon/lm87.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/hwmon/lm87.c b/drivers/hwmon/lm87.c
index c09074f44708..ea23206b3c81 100644
--- a/drivers/hwmon/lm87.c
+++ b/drivers/hwmon/lm87.c
@@ -640,6 +640,7 @@ static ssize_t aout_output_store(struct device *dev,
 {
 	struct i2c_client *client = dev_get_drvdata(dev);
 	struct lm87_data *data = i2c_get_clientdata(client);
+	u8 reg;
 	long val;
 	int err;
 
@@ -647,11 +648,14 @@ static ssize_t aout_output_store(struct device *dev,
 	if (err)
 		return err;
 
+	reg = AOUT_TO_REG(val);
+
 	mutex_lock(&data->update_lock);
-	data->aout = AOUT_TO_REG(val);
-	lm87_write_value(client, LM87_REG_AOUT, data->aout);
+	err = lm87_write_value(client, LM87_REG_AOUT, reg);
+	if (!err)
+		data->aout = reg;
 	mutex_unlock(&data->update_lock);
-	return count;
+	return err < 0 ? err : count;
 }
 static DEVICE_ATTR_RW(aout_output);