Document the SMM interface as requested by Pali Rohar. Since Dell does not offer any offical documentation regarding the SMM interface, the necessary information was extracted from the dell_smm_hwmon driver and other sources. Suggested-by: Pali Rohár <pali@xxxxxxxxxx> Signed-off-by: Armin Wolf <W_Armin@xxxxxx> --- Documentation/hwmon/dell-smm-hwmon.rst | 180 +++++++++++++++++++++++++ 1 file changed, 180 insertions(+) diff --git a/Documentation/hwmon/dell-smm-hwmon.rst b/Documentation/hwmon/dell-smm-hwmon.rst index cfaee682a245..12bba5fd1447 100644 --- a/Documentation/hwmon/dell-smm-hwmon.rst +++ b/Documentation/hwmon/dell-smm-hwmon.rst @@ -173,3 +173,183 @@ obtain the same information and to control the fan status. The ioctl interface can be accessed from C programs or from shell using the i8kctl utility. See the source file of ``i8kutils`` for more information on how to use the ioctl interface. + +SMM Interface +------------- + +.. warning:: The SMM interface was reverse-engineered by trial-and-error + since Dell did not provide any Documentation, + please keep that in mind. + +The driver uses the SMM interface to send commands to the system BIOS. +This interface is normally used by Dell's 32-bit diagnostic program or +on newer notebook models by the buildin BIOS diagnostics. +The SMM is triggered by writing to the special ioports ``0xb2`` and ``0x84``, +and may cause short hangs when the BIOS code is taking too long to +execute. + +The SMM handler inside the system BIOS looks at the contents of the +``eax``, ``ebx``, ``ecx``, ``edx``, ``esi`` and ``edi`` registers. +Each register has a special purpose: + +=============== ================================== +Register Purpose +=============== ================================== +eax Holds the command code before SMM, + holds the first result after SMM. +ebx Holds the arguments. +ecx Unknown, set to 0. +edx Holds the second result after SMM. +esi Unknown, set to 0. +edi Unknown, set to 0. +=============== ================================== + +The SMM handler can signal a failure by either: + +- setting the lower sixteen bits of ``eax`` to ``0xffff`` +- not modifying ``eax`` at all +- setting the carry flag + +SMM command codes +----------------- + +=============== ======================= ================================================ +Command Code Command Name Description +=============== ======================= ================================================ +``0x0025`` Get Fn key status Returns the Fn key pressed after SMM: + + - 9th bit in ``eax`` indicates Volume up + - 10th bit in ``eax`` indicates Volume down + - both bits indicate Volume mute + +``0xa069`` Get power status Returns current power status after SMM: + + - 1st bit in ``eax`` indicates Battery connected + - 3th bit in ``eax`` indicates AC connected + +``0x00a3`` Get fan state Returns current fan state after SMM: + + - 1st byte in ``eax`` holds the current + fan state (0 - 2 or 3) + +``0x01a3`` Set fan state Sets the fan speed: + + - 1st byte in ``ebx`` holds the fan number + - 2nd byte in ``ebx`` holds the desired + fan state (0 - 2 or 3) + +``0x02a3`` Get fan speed Returns the current fan speed in RPM: + + - 1st byte in ``ebx`` holds the fan number + - 1st word in ``eax`` holds the current + fan speed in RPM (after SMM) + +``0x03a3`` Get fan type Returns the fan type: + + - 1st byte in ``ebx`` holds the fan number + - 1st byte in ``eax`` holds the + fan type (after SMM): + + - 5th bit indicates docking fan + - 1 indicates Processor fan + - 2 indicates Motherboard fan + - 3 indicates Video fan + - 4 indicates Power supply fan + - 5 indicates Chipset fan + - 6 indicates other fan type + +``0x04a3`` Get nominal fan speed Returns the nominal RPM in each fan state: + + - 1st byte in ``ebx`` holds the fan number + - 2nd byte in ``ebx`` holds the fan state + in question (0 - 2 or 3) + - 1st word in ``eax`` holds the nominal + fan speed in RPM (after SMM) + +``0x05a3`` Get fan speed tolerance Returns the speed tolerance for each fan state: + + - 1st byte in ``ebx`` holds the fan number + - 2nd byte in ``ebx`` holds the fan state + in question (0 - 2 or 3) + - 1st byte in ``eax`` returns the speed + tolerance + +``0x10a3`` Get sensor temperature Returns the measured temperature: + + - 1st byte in ``ebx`` holds the sensor number + - 1st byte in ``eax`` holds the measured + temperature (after SMM) + +``0x11a3`` Get sensor type Returns the sensor type: + + - 1st byte in ``ebx`` holds the sensor number + - 1st byte in ``eax`` holds the + temperature type (after SMM): + + - 1 indicates CPU sensor + - 2 indicates GPU sensor + - 3 indicates SODIMM sensor + - 4 indicates other sensor type + - 5 indicates Ambient sensor + - 6 indicates other sensor type + +``0xfea3`` Get SMM signature Returns Dell signature if interface + is supported (after SMM): + + - ``eax`` holds 1145651527 + (0x44494147 or "DIAG") + - ``edx`` holds 1145392204 + (0x44454c4c or "DELL") + +``0xffa3`` Get SMM signature Same as ``0xfea3``, check both. +=============== ======================= ================================================ + +There are additional commands for enabling (``0x31a3`` or ``0x35a3``) and +disabling (``0x30a3`` or ``0x34a3``) automatic fan speed control. +The commands are however causing severe sideeffects on many machines, so +they are not used by default. + +On several machines (Inspiron 3505, Precision 490, Vostro 1720, ...), the +fans supports a 4th "magic" state, which signals the BIOS that automatic +fan control should be enabled for a specific fan. +However there are also some machines who do support a 4th regular fan state too, +but in case of the "magic" state, the nominal RPM reported for this state is a +placeholder value, which however is not always detectable. + +Firmware Bugs +------------- + +The SMM calls can behave erratic on some machines: + +======================================================= ================= +Firmware Bug Affected Machines +======================================================= ================= +Reading of fan states return spurious errors. Precision 490 + +Reading of fan types causes erratic fan behaviour. Studio XPS 8000 + + Studio XPS 8100 + + Inspiron 580 + +Fan-related SMM calls take too long (about 500ms). Inspiron 7720 + + Vostro 3360 + + XPS 13 9333 + + XPS 15 L502X +======================================================= ================= + +In case you experience similar issues on your Dell machine, please +submit a bugreport on bugzilla to we can apply workarounds. + +Limitations +----------- + +The SMM calls can take too long to execute on some machines, causing +short hangs and/or audio glitches. +Also the fan state needs to be restored after suspend, as well as +the automatic mode settings. +When reading a temperature sensor, values above 127 degrees indicate +a BIOS read error or a deactivated sensor. -- 2.30.2
