Provide information how to reference I2C busses created by the PIIX4
chip driver from the ACPI code.
Signed-off-by: Konstantin Aladyshev <aladyshev22@xxxxxxxxx>
---
Documentation/i2c/busses/i2c-piix4.rst | 62 ++++++++++++++++++++++++++
1 file changed, 62 insertions(+)
diff --git a/Documentation/i2c/busses/i2c-piix4.rst b/Documentation/i2c/busses/i2c-piix4.rst
index 07fe6f6f4b18..90447dff7419 100644
--- a/Documentation/i2c/busses/i2c-piix4.rst
+++ b/Documentation/i2c/busses/i2c-piix4.rst
@@ -109,3 +109,65 @@ which can easily get corrupted due to a state machine bug. These are mostly
Thinkpad laptops, but desktop systems may also be affected. We have no list
of all affected systems, so the only safe solution was to prevent access to
the SMBus on all IBM systems (detected using DMI data.)
+
+
+Description in the ACPI code
+----------------------------
+
+Device driver for the PIIX4 chip creates a separate I2C bus for each of its ports::
+
+ $ i2cdetect -l
+ ...
+ i2c-7 unknown SMBus PIIX4 adapter port 0 at 0b00 N/A
+ i2c-8 unknown SMBus PIIX4 adapter port 2 at 0b00 N/A
+ i2c-9 unknown SMBus PIIX4 adapter port 1 at 0b20 N/A
+ ...
+
+Therefore if you want to access one of these busses in the ACPI code, port subdevices
+are needed to be declared inside the PIIX device::
+
+ Scope (\_SB_.PCI0.SMBS)
+ {
+ Name (_ADR, 0x00140000)
+
+ Device (SMB0) {
+ Name (_ADR, 0)
+ }
+ Device (SMB1) {
+ Name (_ADR, 1)
+ }
+ Device (SMB2) {
+ Name (_ADR, 2)
+ }
+ }
+
+If it is not the case for your UEFI firmware and you don't have access to the source
+code, you can use ACPI SSDT Overlays to provide the missing parts. Just keep in mind
+that in this case you would need to load your extra SSDT table before the piix4 driver
+start, i.e. you should provide SSDT via initrd or EFI variable methods and not via
+configfs.
+
+As an example of usage here is the ACPI snippet code that would assign jc42 driver
+to the 0x1C device on the I2C bus created by the PIIX port 0::
+
+ Device (JC42) {
+ Name (_HID, "PRP0001")
+ Name (_DDN, "JC42 Temperature sensor")
+ Name (_CRS, ResourceTemplate () {
+ I2cSerialBusV2 (
+ 0x001c,
+ ControllerInitiated,
+ 100000,
+ AddressingMode7Bit,
+ "\\_SB.PCI0.SMBS.SMB0",
+ 0
+ )
+ })
+
+ Name (_DSD, Package () {
+ ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+ Package () {
+ Package () { "compatible", Package() { "jedec,jc-42.4-temp" } },
+ }
+ })
+ }
--
2.43.0
