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docs: i2c: piix4: Add ACPI section

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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>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
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Konstantin Aladyshev 2024-11-11 17:02:31 +03:00 committed by Andi Shyti
parent c366be7202
commit 1922bc2455
1 changed files with 63 additions and 0 deletions
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63
Documentation/i2c/busses/i2c-piix4.rst
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@ -109,3 +109,66 @@ 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 this 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 starts, 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" } },
}
})
}