From ed7fcf1ed5ea4ea01243995ae085757a77cf0f3e Mon Sep 17 00:00:00 2001 From: Andy Shevchenko Date: Tue, 23 May 2017 20:03:21 +0300 Subject: 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 Signed-off-by: Andy Shevchenko Tested-by: Jarkko Nikula Reviewed-by: Mika Westerberg Signed-off-by: Linus Walleij --- Documentation/acpi/gpio-properties.txt | 65 ++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) (limited to 'Documentation/acpi') diff --git a/Documentation/acpi/gpio-properties.txt b/Documentation/acpi/gpio-properties.txt index 2aff0349facd..88c65cb5bf0a 100644 --- a/Documentation/acpi/gpio-properties.txt +++ b/Documentation/acpi/gpio-properties.txt @@ -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. -- cgit v1.2.3-59-g8ed1b