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hwmon: (dell-smm) Document the WMI SMM interface 

Document the WMI SMM interface so that future developers
can better understand how it works.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20231123004820.50635-9-W_Armin@gmx.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
This commit is contained in:
Armin Wolf 2023-11-23 01:48:19 +01:00 committed by Guenter Roeck
parent b7a4706f66
commit e7caf3d1e8
1 changed files with 35 additions and 3 deletions
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38
Documentation/hwmon/dell-smm-hwmon.rst
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@ -186,8 +186,7 @@ SMM Interface
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
The SMM 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
@ -210,7 +209,40 @@ 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
- setting the carry flag (legacy SMM interface only)
Legacy SMM Interface
--------------------
When using the legacy SMM interface, a SMM is triggered by writing the least significant byte
of the command code to the special ioports ``0xb2`` and ``0x84``. This interface is not
described inside the ACPI tables and can thus only be detected by issuing a test SMM call.
WMI SMM Interface
-----------------
On modern Dell machines, the SMM calls are done over ACPI WMI:
::
#pragma namespace("\\\\.\\root\\dcim\\sysman\\diagnostics")
[WMI, Provider("Provider_DiagnosticsServices"), Dynamic, Locale("MS\\0x409"),
Description("RunDellDiag"), guid("{F1DDEE52-063C-4784-A11E-8A06684B9B01}")]
class LegacyDiags {
[key, read] string InstanceName;
[read] boolean Active;
[WmiMethodId(1), Implemented, read, write, Description("Legacy Method ")]
void Execute([in, out] uint32 EaxLen, [in, out, WmiSizeIs("EaxLen") : ToInstance] uint8 EaxVal[],
[in, out] uint32 EbxLen, [in, out, WmiSizeIs("EbxLen") : ToInstance] uint8 EbxVal[],
[in, out] uint32 EcxLen, [in, out, WmiSizeIs("EcxLen") : ToInstance] uint8 EcxVal[],
[in, out] uint32 EdxLen, [in, out, WmiSizeIs("EdxLen") : ToInstance] uint8 EdxVal[]);
};
Some machines support only the WMI SMM interface, while some machines support both interfaces.
The driver automatically detects which interfaces are present and will use the WMI SMM interface
if the legacy SMM interface is not present. The WMI SMM interface is usually slower than the
legacy SMM interface since ACPI methods need to be called in order to trigger a SMM.
SMM command codes
-----------------