gpiolib: Add support for GPIO lookup by line name

Currently a GPIO lookup table can only refer to a specific GPIO by a
tuple, consisting of a GPIO controller label and a GPIO offset inside
the controller.

However, a GPIO may also carry a line name, defined by DT or ACPI.
If present, the line name is the most use-centric way to refer to a
GPIO.  Hence add support for looking up GPIOs by line name.
Note that there is no guarantee that GPIO line names are globally
unique, so this will use the first match found.

Implement this by reusing the existing gpiod_lookup infrastructure.
Rename gpiod_lookup.chip_label to gpiod_lookup.key, to make it clear
that this field can have two meanings, and update the kerneldoc and
GPIO_LOOKUP*() macros.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20200511145257.22970-4-geert+renesas@glider.be
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/Documentation/driver-api/gpio/board.rst b/Documentation/driver-api/gpio/board.rst
index ce91518bf9f4..191fa867826a 100644
--- a/Documentation/driver-api/gpio/board.rst
+++ b/Documentation/driver-api/gpio/board.rst
@@ -113,13 +113,15 @@ files that desire to do so need to include the following header::
 GPIOs are mapped by the means of tables of lookups, containing instances of the
 gpiod_lookup structure. Two macros are defined to help declaring such mappings::
 
-	GPIO_LOOKUP(chip_label, chip_hwnum, con_id, flags)
-	GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags)
+	GPIO_LOOKUP(key, chip_hwnum, con_id, flags)
+	GPIO_LOOKUP_IDX(key, chip_hwnum, con_id, idx, flags)
 
 where
 
-  - chip_label is the label of the gpiod_chip instance providing the GPIO
-  - chip_hwnum is the hardware number of the GPIO within the chip
+  - key is either the label of the gpiod_chip instance providing the GPIO, or
+    the GPIO line name
+  - chip_hwnum is the hardware number of the GPIO within the chip, or U16_MAX
+    to indicate that key is a GPIO line name
   - con_id is the name of the GPIO function from the device point of view. It
 	can be NULL, in which case it will match any function.
   - idx is the index of the GPIO within the function.
@@ -135,7 +137,10 @@ where
 
 In the future, these flags might be extended to support more properties.
 
-Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
+Note that:
+  1. GPIO line names are not guaranteed to be globally unique, so the first
+     match found will be used.
+  2. GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
 
 A lookup table can then be defined as follows, with an empty entry defining its
 end. The 'dev_id' field of the table is the identifier of the device that will