[PATCH] docs: i2c: piix4: Add ACPI section

[Konstantin Aladyshev <aladyshev22@gmail.com>]

Provide information how to reference I2C busses created by the PIIX4
chip driver from the ACPI code.

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
---
 Documentation/i2c/busses/i2c-piix4.rst | 56 ++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/Documentation/i2c/busses/i2c-piix4.rst b/Documentation/i2c/busses/i2c-piix4.rst
index 07fe6f6f4b18..2a00158b508a 100644
--- a/Documentation/i2c/busses/i2c-piix4.rst
+++ b/Documentation/i2c/busses/i2c-piix4.rst
@@ -109,3 +109,59 @@ 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, you need to
+declare port subdevices 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)
+        }
+    }
+
+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