gpio: acpi: Explain how to get GPIO descriptors in ACPI case
Documentation lacks of explanation how we actually use device properties for GPIO resources. Add a section to the documentation about that. Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com> Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
parent
6fe9da42f1
commit
ed7fcf1ed5
|
@ -156,3 +156,68 @@ pointed to by its first argument. That should be done in the driver's .probe()
|
|||
routine. On removal, the driver should unregister its GPIO mapping table by
|
||||
calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
|
||||
table was previously registered.
|
||||
|
||||
Using the _CRS fallback
|
||||
-----------------------
|
||||
|
||||
If a device does not have _DSD or the driver does not create ACPI GPIO
|
||||
mapping, the Linux GPIO framework refuses to return any GPIOs. This is
|
||||
because the driver does not know what it actually gets. For example if we
|
||||
have a device like below:
|
||||
|
||||
Device (BTH)
|
||||
{
|
||||
Name (_HID, ...)
|
||||
|
||||
Name (_CRS, ResourceTemplate () {
|
||||
GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
|
||||
"\\_SB.GPO0", 0, ResourceConsumer) {15}
|
||||
GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
|
||||
"\\_SB.GPO0", 0, ResourceConsumer) {27}
|
||||
})
|
||||
}
|
||||
|
||||
The driver might expect to get the right GPIO when it does:
|
||||
|
||||
desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
|
||||
|
||||
but since there is no way to know the mapping between "reset" and
|
||||
the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
|
||||
|
||||
The driver author can solve this by passing the mapping explictly
|
||||
(the recommended way and documented in the above chapter).
|
||||
|
||||
The ACPI GPIO mapping tables should not contaminate drivers that are not
|
||||
knowing about which exact device they are servicing on. It implies that
|
||||
the ACPI GPIO mapping tables are hardly linked to ACPI ID and certain
|
||||
objects, as listed in the above chapter, of the device in question.
|
||||
|
||||
Getting GPIO descriptor
|
||||
-----------------------
|
||||
|
||||
There are two main approaches to get GPIO resource from ACPI:
|
||||
desc = gpiod_get(dev, connection_id, flags);
|
||||
desc = gpiod_get_index(dev, connection_id, index, flags);
|
||||
|
||||
We may consider two different cases here, i.e. when connection ID is
|
||||
provided and otherwise.
|
||||
|
||||
Case 1:
|
||||
desc = gpiod_get(dev, "non-null-connection-id", flags);
|
||||
desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
|
||||
|
||||
Case 2:
|
||||
desc = gpiod_get(dev, NULL, flags);
|
||||
desc = gpiod_get_index(dev, NULL, index, flags);
|
||||
|
||||
Case 1 assumes that corresponding ACPI device description must have
|
||||
defined device properties and will prevent to getting any GPIO resources
|
||||
otherwise.
|
||||
|
||||
Case 2 explicitly tells GPIO core to look for resources in _CRS.
|
||||
|
||||
Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
|
||||
are two versions of ACPI device description provided and no mapping is
|
||||
present in the driver, will return different resources. That's why a
|
||||
certain driver has to handle them carefully as explained in previous
|
||||
chapter.
|
||||
|
|
Loading…
Reference in New Issue
Block a user