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72cef64180
Setting GPIO direction = high, sometimes results in GPIO value = 0.
If a GPIO is pulled high, the following construction results in the
value being 0 when the desired value is 1:
$ echo "high" > /sys/class/gpio/gpio336/direction
$ cat /sys/class/gpio/gpio336/value
0
Before the GPIO direction is changed from an input to an output,
exar_set_value() is called with value = 1, but since the GPIO is an
input when exar_set_value() is called, _regmap_update_bits() reads a 1
due to an external pull-up. regmap_set_bits() sets force_write =
false, so the value (1) is not written. When the direction is then
changed, the GPIO becomes an output with the value of 0 (the hardware
default).
regmap_write_bits() sets force_write = true, so the value is always
written by exar_set_value() and an external pull-up doesn't affect the
outcome of setting direction = high.
The same can happen when a GPIO is pulled low, but the scenario is a
little more complicated.
$ echo high > /sys/class/gpio/gpio351/direction
$ cat /sys/class/gpio/gpio351/value
1
$ echo in > /sys/class/gpio/gpio351/direction
$ cat /sys/class/gpio/gpio351/value
0
$ echo low > /sys/class/gpio/gpio351/direction
$ cat /sys/class/gpio/gpio351/value
1
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.