vt1211: Add documentation

vt1211: Add documentation

Add documentation for the new vt1211 hardware monitoring driver.
Signed-off-by: Juerg Haefliger <juergh@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/hwmon/vt1211

+Kernel driver vt1211
+====================
+
+Supported chips:
+  * VIA VT1211
+    Prefix: 'vt1211'
+    Addresses scanned: none, address read from Super-I/O config space
+    Datasheet: Provided by VIA upon request and under NDA
+
+Authors: Juerg Haefliger <juergh@gmail.com>
+
+This driver is based on the driver for kernel 2.4 by Mark D. Studebaker and
+its port to kernel 2.6 by Lars Ekman.
+
+Thanks to Joseph Chan and Fiona Gatt from VIA for providing documentation and
+technical support.
+
+
+Description
+-----------
+
+The VIA VT1211 Super-I/O chip includes complete hardware monitoring
+capabilities. It monitors 2 dedicated temperature sensor inputs (temp1 and
+temp2), 1 dedicated voltage (in5) and 2 fans. Additionally, the chip
+implements 5 universal input channels (UCH1-5) that can be individually
+programmed to either monitor a voltage or a temperature.
+
+This chip also provides manual and automatic control of fan speeds (according
+to the datasheet). The driver only supports automatic control since the manual
+mode doesn't seem to work as advertised in the datasheet. In fact I couldn't
+get manual mode to work at all! Be aware that automatic mode hasn't been
+tested very well (due to the fact that my EPIA M10000 doesn't have the fans
+connected to the PWM outputs of the VT1211 :-().
+
+The following table shows the relationship between the vt1211 inputs and the
+sysfs nodes.
+
+Sensor          Voltage Mode   Temp Mode   Default Use (from the datasheet)
+------          ------------   ---------   --------------------------------
+Reading 1                      temp1       Intel thermal diode
+Reading 3                      temp2       Internal thermal diode
+UCH1/Reading2   in0            temp3       NTC type thermistor
+UCH2            in1            temp4       +2.5V
+UCH3            in2            temp5       VccP (processor core)
+UCH4            in3            temp6       +5V
+UCH5            in4            temp7       +12V
++3.3V           in5                        Internal VCC (+3.3V)
+
+
+Voltage Monitoring
+------------------
+
+Voltages are sampled by an 8-bit ADC with a LSB of ~10mV. The supported input
+range is thus from 0 to 2.60V. Voltage values outside of this range need
+external scaling resistors. This external scaling needs to be compensated for
+via compute lines in sensors.conf, like:
+
+compute inx @*(1+R1/R2), @/(1+R1/R2)
+
+The board level scaling resistors according to VIA's recommendation are as
+follows. And this is of course totally dependent on the actual board
+implementation :-) You will have to find documentation for your own
+motherboard and edit sensors.conf accordingly.
+
+                                      Expected
+Voltage       R1     R2     Divider   Raw Value
+-----------------------------------------------
++2.5V         2K     10K    1.2       2083 mV
+VccP          ---    ---    1.0       1400 mV (1)
++5V           14K    10K    2.4       2083 mV
++12V          47K    10K    5.7       2105 mV
++3.3V (int)   2K     3.4K   1.588     3300 mV (2)
++3.3V (ext)   6.8K   10K    1.68      1964 mV
+
+(1) Depending on the CPU (1.4V is for a VIA C3 Nehemiah).
+(2) R1 and R2 for 3.3V (int) are internal to the VT1211 chip and the driver
+    performs the scaling and returns the properly scaled voltage value.
+
+Each measured voltage has an associated low and high limit which triggers an
+alarm when crossed.
+
+
+Temperature Monitoring
+----------------------
+
+Temperatures are reported in millidegree Celsius. Each measured temperature
+has a high limit which triggers an alarm if crossed. There is an associated
+hysteresis value with each temperature below which the temperature has to drop
+before the alarm is cleared (this is only true for interrupt mode 0). The
+interrupt mode can be forced to 0 in case the BIOS doesn't do it
+automatically. For that, the parameter int_mode=0 needs to be supplied when
+loading the driver module. Be aware that overriding BIOS defaults might cause
+some unwanted side effects!
+
+All temperature channels except temp2 are external. Temp2 is the VT1211
+internal thermal diode and the driver does all the scaling for temp2 and
+returns the temperature in millidegree Celsius. For the external channels
+temp1 and temp3-temp7, scaling depends on the board implementation and needs
+to be performed in userspace via sensors.conf.
+
+Temp1 is an Intel-type thermal diode which requires the following formula to
+convert between sysfs readings and real temperatures:
+
+compute temp1 (@-Offset)/Gain, (@*Gain)+Offset
+
+According to the VIA VT1211 BIOS porting guide, the following gain and offset
+values should be used:
+
+Diode Type      Offset   Gain
+----------      ------   ----
+Intel CPU       88.638   0.9528
+                65.000   0.9686   *)
+VIA C3 Ezra     83.869   0.9528
+VIA C3 Ezra-T   73.869   0.9528
+
+*) This is the formula from the lm_sensors 2.10.0 sensors.conf file. I don't
+know where it comes from or how it was derived, it's just listed here for
+completeness.
+
+Temp3-temp7 support NTC thermistors. For these channels, the driver returns
+the voltages as seen at the individual pins of UCH1-UCH5. The voltage at the
+pin (Vpin) is formed by a voltage divider made of the thermistor (Rth) and a
+scaling resistor (Rs):
+
+Vpin = 2200 * Rth / (Rs + Rth)   (2200 is the ADC max limit of 2200 mV)
+
+The equation for the thermistor is as follows (google it if you want to know
+more about it):
+
+Rth = Ro * exp(B * (1 / T - 1 / To))   (To is 298.15K (25C) and Ro is the
+                                        nominal resistance at 25C)
+
+Mingling the above two equations and assuming Rs = Ro and B = 3435 yields the
+following formula for sensors.conf:
+
+compute tempx 1 / (1 / 298.15 - (` (2200 / @ - 1)) / 3435) - 273.15,
+              2200 / (1 + (^ (3435 / 298.15 - 3435 / (273.15 + @))))
+
+
+Fan Speed Control
+-----------------
+
+The VT1211 provides 2 programmable PWM outputs to control the speeds of 2
+fans. Writing a 2 to any of the two pwm[1-2]_enable sysfs nodes will put the
+PWM controller in automatic mode. There is only a single controller that
+controls both PWM outputs but each PWM output can be individually enabled and
+disabled.
+
+Each PWM has 4 associated distinct output duty-cycles: full, high, low and
+off. Full and off are internally hard-wired to 255 (100%) and 0 (0%),
+respectively. High and low can be programmed via
+pwm[1-2]_auto_point[2-3]_pwm. Each PWM output can be associated with a
+different thermal input but - and here's the weird part - only one set of
+thermal thresholds exist that controls both PWMs output duty-cycles. The
+thermal thresholds are accessible via pwm[1-2]_auto_point[1-4]_temp. Note
+that even though there are 2 sets of 4 auto points each, they map to the same
+registers in the VT1211 and programming one set is sufficient (actually only
+the first set pwm1_auto_point[1-4]_temp is writable, the second set is
+read-only).
+
+PWM Auto Point             PWM Output Duty-Cycle
+------------------------------------------------
+pwm[1-2]_auto_point4_pwm   full speed duty-cycle (hard-wired to 255)
+pwm[1-2]_auto_point3_pwm   high speed duty-cycle
+pwm[1-2]_auto_point2_pwm   low speed duty-cycle
+pwm[1-2]_auto_point1_pwm   off duty-cycle (hard-wired to 0)
+
+Temp Auto Point             Thermal Threshold
+---------------------------------------------
+pwm[1-2]_auto_point4_temp   full speed temp
+pwm[1-2]_auto_point3_temp   high speed temp
+pwm[1-2]_auto_point2_temp   low speed temp
+pwm[1-2]_auto_point1_temp   off temp
+
+Long story short, the controller implements the following algorithm to set the
+PWM output duty-cycle based on the input temperature:
+
+Thermal Threshold             Output Duty-Cycle
+                    (Rising Temp)           (Falling Temp)
+----------------------------------------------------------
+                    full speed duty-cycle   full speed duty-cycle
+full speed temp
+                    high speed duty-cycle   full speed duty-cycle
+high speed temp
+                    low speed duty-cycle    high speed duty-cycle
+low speed temp
+                    off duty-cycle          low speed duty-cycle
+off temp