thermal: sysfs: Return ENODATA instead of EAGAIN for reads

According to POSIX spec, EAGAIN returned by read with O_NONBLOCK set
means the read would block. Hence, the common implementation in
nonblocking model will poll the file when the nonblocking read returns
EAGAIN. However, when the target file is thermal zone, this mechanism
will totally malfunction because thermal zone doesn't implement sysfs
notification and thus the poll will never return.

For example, the read in Golang implemnts such method and sometimes
hangs at reading some thermal zones via sysfs.

Change to return -ENODATA instead of -EAGAIN to userspace.

Signed-off-by: Hsin-Te Yuan <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Hsin-Te Yuan

drivers/thermal/thermal_sysfs.c

@@ -40,10 +40,13 @@ temp_show(struct device *dev, struct device_attribute *attr, char *buf)
 
 	ret = thermal_zone_get_temp(tz, &temperature);
 
-	if (ret)
-		return ret;
+	if (!ret)
+		return sprintf(buf, "%d\n", temperature);
 
-	return sprintf(buf, "%d\n", temperature);
+	if (ret == -EAGAIN)
+		return -ENODATA;
+
+	return ret;
 }
 
 static ssize_t