gpiolib: Implement fast processing path in get/set array

Certain GPIO descriptor arrays returned by gpio_get_array() may contain
information on direct mapping of array members to pins of a single GPIO
chip in hardware order.  In such cases, bitmaps of values can be passed
directly from/to the chip's .get/set_multiple() callbacks without
wasting time on iterations.

Add respective code to gpiod_get/set_array_bitmap_complex() functions.
Pins not applicable for fast path are processed as before, skipping
over the 'fast' ones.

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/Documentation/driver-api/gpio/board.rst b/Documentation/driver-api/gpio/board.rst
index 2c112553df84..c66821e033c2 100644
--- a/Documentation/driver-api/gpio/board.rst
+++ b/Documentation/driver-api/gpio/board.rst
@@ -193,3 +193,18 @@ And the table can be added to the board code as follows::
 
 The line will be hogged as soon as the gpiochip is created or - in case the
 chip was created earlier - when the hog table is registered.
+
+Arrays of pins
+--------------
+In addition to requesting pins belonging to a function one by one, a device may
+also request an array of pins assigned to the function.  The way those pins are
+mapped to the device determines if the array qualifies for fast bitmap
+processing.  If yes, a bitmap is passed over get/set array functions directly
+between a caller and a respective .get/set_multiple() callback of a GPIO chip.
+
+In order to qualify for fast bitmap processing, the pin mapping must meet the
+following requirements:
+- it must belong to the same chip as other 'fast' pins of the function,
+- its index within the function must match its hardware number within the chip.
+
+Open drain and open source pins are excluded from fast bitmap output processing.