Documentation/userspace-api/gpio/gpio-get-linehandle-ioctl.rst

.. SPDX-License-Identifier: GPL-2.0

.. _GPIO_GET_LINEHANDLE_IOCTL:

*************************
GPIO_GET_LINEHANDLE_IOCTL
*************************

.. warning::
    This ioctl is part of chardev_v1.rst and is obsoleted by
    gpio-v2-get-line-ioctl.rst.

Name
====

GPIO_GET_LINEHANDLE_IOCTL - Request a line or lines from the kernel.

Synopsis
========

.. c:macro:: GPIO_GET_LINEHANDLE_IOCTL

``int ioctl(int chip_fd, GPIO_GET_LINEHANDLE_IOCTL, struct gpiohandle_request *request)``

Arguments
=========

``chip_fd``
    The file descriptor of the GPIO character device returned by `open()`.

``request``
    The :c:type:`handle_request<gpiohandle_request>` specifying the lines to
    request and their configuration.

Description
===========

Request a line or lines from the kernel.

While multiple lines may be requested, the same configuration applies to all
lines in the request.

On success, the requesting process is granted exclusive access to the line
value and write access to the line configuration.

The state of a line, including the value of output lines, is guaranteed to
remain as requested until the returned file descriptor is closed. Once the
file descriptor is closed, the state of the line becomes uncontrolled from
the userspace perspective, and may revert to its default state.

Requesting a line already in use is an error (**EBUSY**).

Closing the ``chip_fd`` has no effect on existing line handles.

.. _gpio-get-linehandle-config-rules:

Configuration Rules
-------------------

The following configuration rules apply:

The direction flags, ``GPIOHANDLE_REQUEST_INPUT`` and
``GPIOHANDLE_REQUEST_OUTPUT``, cannot be combined. If neither are set then the
only other flag that may be set is ``GPIOHANDLE_REQUEST_ACTIVE_LOW`` and the
line is requested "as-is" to allow reading of the line value without altering
the electrical configuration.

The drive flags, ``GPIOHANDLE_REQUEST_OPEN_xxx``, require the
``GPIOHANDLE_REQUEST_OUTPUT`` to be set.
Only one drive flag may be set.
If none are set then the line is assumed push-pull.

Only one bias flag, ``GPIOHANDLE_REQUEST_BIAS_xxx``, may be set, and
it requires a direction flag to also be set.
If no bias flags are set then the bias configuration is not changed.

Requesting an invalid configuration is an error (**EINVAL**).


.. _gpio-get-linehandle-config-support:

Configuration Support
---------------------

Where the requested configuration is not directly supported by the underlying
hardware and driver, the kernel applies one of these approaches:

 - reject the request
 - emulate the feature in software
 - treat the feature as best effort

The approach applied depends on whether the feature can reasonably be emulated
in software, and the impact on the hardware and userspace if the feature is not
supported.
The approach applied for each feature is as follows:

==============   ===========
Feature          Approach
==============   ===========
Bias             best effort
Direction        reject
Drive            emulate
==============   ===========

Bias is treated as best effort to allow userspace to apply the same
configuration for platforms that support internal bias as those that require
external bias.
Worst case the line floats rather than being biased as expected.

Drive is emulated by switching the line to an input when the line should not
be driven.

In all cases, the configuration reported by gpio-get-lineinfo-ioctl.rst
is the requested configuration, not the resulting hardware configuration.
Userspace cannot determine if a feature is supported in hardware, is
emulated, or is best effort.

Return Value
============

On success 0 and the :c:type:`request.fd<gpiohandle_request>` contains the
file descriptor for the request.

On error -1 and the ``errno`` variable is set appropriately.
Common error codes are described in error-codes.rst.