xref: /linux/Documentation/userspace-api/gpio/gpio-v2-get-line-ioctl.rst (revision a261e208)

.. SPDX-License-Identifier: GPL-2.0

.. _GPIO_V2_GET_LINE_IOCTL:

**********************
GPIO_V2_GET_LINE_IOCTL
**********************

Name
====

GPIO_V2_GET_LINE_IOCTL - Request a line or lines from the kernel.

Synopsis
========

.. c:macro:: GPIO_V2_GET_LINE_IOCTL

``int ioctl(int chip_fd, GPIO_V2_GET_LINE_IOCTL, struct gpio_v2_line_request *request)``

Arguments
=========

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

``request``
    The :c:type:`line_request<gpio_v2_line_request>` specifying the lines
    to request and their configuration.

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

On success, the requesting process is granted exclusive access to the line
value, write access to the line configuration, and may receive events when
edges are detected on the line, all of which are described in more detail in
:ref:`gpio-v2-line-request`.

A number of lines may be requested in the one line request, and request
operations are performed on the requested lines by the kernel as atomically
as possible. e.g. gpio-v2-line-get-values-ioctl.rst will read all the
requested lines at once.

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 requests.

.. _gpio-v2-get-line-config-rules:

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

For any given requested line, the following configuration rules apply:

The direction flags, ``GPIO_V2_LINE_FLAG_INPUT`` and
``GPIO_V2_LINE_FLAG_OUTPUT``, cannot be combined. If neither are set then
the only other flag that may be set is ``GPIO_V2_LINE_FLAG_ACTIVE_LOW``
and the line is requested "as-is" to allow reading of the line value
without altering the electrical configuration.

The drive flags, ``GPIO_V2_LINE_FLAG_OPEN_xxx``, require the
``GPIO_V2_LINE_FLAG_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, ``GPIO_V2_LINE_FLAG_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.

The edge flags, ``GPIO_V2_LINE_FLAG_EDGE_xxx``, require
``GPIO_V2_LINE_FLAG_INPUT`` to be set and may be combined to detect both rising
and falling edges.  Requesting edge detection from a line that does not support
it is an error (**ENXIO**).

Only one event clock flag, ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_xxx``, may be set.
If none are set then the event clock defaults to ``CLOCK_MONOTONIC``.
The ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE`` flag requires supporting hardware
and a kernel with ``CONFIG_HTE`` set.  Requesting HTE from a device that
doesn't support it is an error (**EOPNOTSUPP**).

The :c:type:`debounce_period_us<gpio_v2_line_attribute>` attribute may only
be applied to lines with ``GPIO_V2_LINE_FLAG_INPUT`` set. When set, debounce
applies to both the values returned by gpio-v2-line-get-values-ioctl.rst and
the edges returned by gpio-v2-line-event-read.rst.  If not
supported directly by hardware, debouncing is emulated in software by the
kernel.  Requesting debounce on a line that supports neither debounce in
hardware nor interrupts, as required for software emulation, is an error
(**ENXIO**).

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

.. _gpio-v2-get-line-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
Debounce         emulate
Direction        reject
Drive            emulate
Edge Detection   reject
==============   ===========

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.

Debounce is emulated by applying a filter to hardware interrupts on the line.
An edge event is generated after an edge is detected and the line remains
stable for the debounce period.
The event timestamp corresponds to the end of the debounce period.

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

Edge detection requires interrupt support, and is rejected if that is not
supported. Emulation by polling can still be performed from userspace.

In all cases, the configuration reported by gpio-v2-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<gpio_v2_line_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.