w1: w1_therm: Add force-pullup option for "broken" sensors

Signed-off-by: Michael Arndt <michael@scriptkiller.de>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/w1/slaves/w1_therm

@@ -34,9 +34,16 @@ currently supported.  The driver also doesn't support reduced
 precision (which would also reduce the conversion time).
 
 The module parameter strong_pullup can be set to 0 to disable the
-strong pullup or 1 to enable.  If enabled the 5V strong pullup will be
-enabled when the conversion is taking place provided the master driver
-must support the strong pullup (or it falls back to a pullup
+strong pullup, 1 to enable autodetection or 2 to force strong pullup.
+In case of autodetection, the driver will use the "READ POWER SUPPLY"
+command to check if there are pariste powered devices on the bus.
+If so, it will activate the master's strong pullup.
+In case the detection of parasite devices using this command fails
+(seems to be the case with some DS18S20) the strong pullup can
+be force-enabled.
+If the strong pullup is enabled, the master's strong pullup will be
+driven when the conversion is taking place, provided the master driver
+does support the strong pullup (or it falls back to a pullup
 resistor).  The DS18b20 temperature sensor specification lists a
 maximum current draw of 1.5mA and that a 5k pullup resistor is not
 sufficient.  The strong pullup is designed to provide the additional

drivers/w1/slaves/w1_therm.c

@@ -41,6 +41,14 @@ MODULE_DESCRIPTION("Driver for 1-wire Dallas network protocol, temperature famil
  * If it was disabled a parasite powered device might not get the require
  * current to do a temperature conversion.  If it is enabled parasite powered
  * devices have a better chance of getting the current required.
+ * In case the parasite power-detection is not working (seems to be the case
+ * for some DS18S20) the strong pullup can also be forced, regardless of the
+ * power state of the devices.
+ *
+ * Summary of options:
+ * - strong_pullup = 0	Disable strong pullup completely
+ * - strong_pullup = 1	Enable automatic strong pullup detection
+ * - strong_pullup = 2	Force strong pullup
  */
 static int w1_strong_pullup = 1;
 module_param_named(strong_pullup, w1_strong_pullup, int, 0);
@@ -197,7 +205,8 @@ static ssize_t w1_therm_read(struct device *device,
 				continue;
 
 			/* 750ms strong pullup (or delay) after the convert */
-			if (!external_power && w1_strong_pullup)
+			if (w1_strong_pullup == 2 ||
+					(!external_power && w1_strong_pullup))
 				w1_next_pullup(dev, tm);
 
 			w1_write_8(dev, W1_CONVERT_TEMP);