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Documentation/drivers: Add section for i2c detect feature 

Add a section explaining how the detect feature works and when it should
be used.

Change-Id: Ife5178d4565e76d0ff436c835c9c47525015c3ed
Signed-off-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67763
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
This commit is contained in:
Matt DeVillier 2022-09-21 10:33:22 -05:00 committed by Felix Held
parent 72bae5b9bd
commit 0208b51215
1 changed files with 30 additions and 2 deletions
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32
Documentation/drivers/dt_entries.md
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@ -9,6 +9,7 @@ device pci 15.0 on
register "hid" = ""ELAN0000""
register "desc" = ""ELAN Touchpad""
register "irq" = "ACPI_IRQ_WAKE_LEVEL_LOW(GPP_A21_IRQ)"
register "detect" = "1"
register "wake" = "GPE0_DW0_21"
device i2c 15 on end
end
@ -140,6 +141,31 @@ find the names in your SoC's header file. The ACPI_* macros are defined in
Using a GPIO as an IRQ requires that it is configured in coreboot correctly.
This is often done in a mainboard-specific file named ``gpio.c``.
### detect
The next register is:
```
register "detect" = "1"
```
This flag tells the I2C driver that it should attempt to detect the presence of
the device (using an I2C zero-byte write), and only generate a SSDT entry if the
device is actually present. This alleviates the OS from having to determine if
a device is present or not (ChromeOS/Linux) and prevents resource conflict/
driver issues (Windows).
Currently, the detect feature works and is hooked up for all I2C touchpads,
and should be used any time a board has multiple touchpad options.
I2C audio devices should also work without issue.
Touchscreens can use this feature as well, but special care is needed to
implement the proper power sequencing for the device to be detected. Generally,
this means driving the enable GPIO high and holding the reset GPIO low in early
GPIO init (bootblock/romstage), then releasing reset in ramstage. While no
boards in the tree currently implement this, it has been used in downstream
forks without issue for some time now.
### wake
The last register is:
@ -206,5 +232,7 @@ Name (_CRS, ResourceTemplate () // _CRS: Current Resource Settings
## Notes
- **All devices in devicetrees end up in the SSDT table, and are generated in
coreboot's ramstage**
- **All device driver entries in devicetrees end up in the SSDT table, and are
generated in coreboot's ramstage**
(The lone exception to this rule is i2c touchpads with the 'detect' flag set;
in this case, devices not present will not be added to the SSDT)