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The existing uAPI documentation does not adequately describe how the kernel handles the case where the underlying hardware or driver does not support the requested configuration. Add a Configuration Support section describing that behaviour to both the v1 and v2 documentation, and better document the errors returned where the requested configuration cannot be supported. Signed-off-by: Kent Gibson <warthog618@gmail.com> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
126 lines
3.8 KiB
ReStructuredText
126 lines
3.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _GPIO_GET_LINEHANDLE_IOCTL:
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*************************
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GPIO_GET_LINEHANDLE_IOCTL
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*************************
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.. warning::
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This ioctl is part of chardev_v1.rst and is obsoleted by
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gpio-v2-get-line-ioctl.rst.
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Name
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====
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GPIO_GET_LINEHANDLE_IOCTL - Request a line or lines from the kernel.
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Synopsis
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========
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.. c:macro:: GPIO_GET_LINEHANDLE_IOCTL
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``int ioctl(int chip_fd, GPIO_GET_LINEHANDLE_IOCTL, struct gpiohandle_request *request)``
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Arguments
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=========
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``chip_fd``
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The file descriptor of the GPIO character device returned by `open()`.
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``request``
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The :c:type:`handle_request<gpiohandle_request>` specifying the lines to
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request and their configuration.
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Description
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===========
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Request a line or lines from the kernel.
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While multiple lines may be requested, the same configuration applies to all
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lines in the request.
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On success, the requesting process is granted exclusive access to the line
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value and write access to the line configuration.
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The state of a line, including the value of output lines, is guaranteed to
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remain as requested until the returned file descriptor is closed. Once the
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file descriptor is closed, the state of the line becomes uncontrolled from
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the userspace perspective, and may revert to its default state.
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Requesting a line already in use is an error (**EBUSY**).
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Closing the ``chip_fd`` has no effect on existing line handles.
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.. _gpio-get-linehandle-config-rules:
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Configuration Rules
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-------------------
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The following configuration rules apply:
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The direction flags, ``GPIOHANDLE_REQUEST_INPUT`` and
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``GPIOHANDLE_REQUEST_OUTPUT``, cannot be combined. If neither are set then the
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only other flag that may be set is ``GPIOHANDLE_REQUEST_ACTIVE_LOW`` and the
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line is requested "as-is" to allow reading of the line value without altering
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the electrical configuration.
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The drive flags, ``GPIOHANDLE_REQUEST_OPEN_xxx``, require the
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``GPIOHANDLE_REQUEST_OUTPUT`` to be set.
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Only one drive flag may be set.
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If none are set then the line is assumed push-pull.
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Only one bias flag, ``GPIOHANDLE_REQUEST_BIAS_xxx``, may be set, and
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it requires a direction flag to also be set.
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If no bias flags are set then the bias configuration is not changed.
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Requesting an invalid configuration is an error (**EINVAL**).
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.. _gpio-get-linehandle-config-support:
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Configuration Support
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---------------------
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Where the requested configuration is not directly supported by the underlying
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hardware and driver, the kernel applies one of these approaches:
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- reject the request
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- emulate the feature in software
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- treat the feature as best effort
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The approach applied depends on whether the feature can reasonably be emulated
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in software, and the impact on the hardware and userspace if the feature is not
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supported.
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The approach applied for each feature is as follows:
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============== ===========
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Feature Approach
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============== ===========
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Bias best effort
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Direction reject
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Drive emulate
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============== ===========
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Bias is treated as best effort to allow userspace to apply the same
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configuration for platforms that support internal bias as those that require
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external bias.
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Worst case the line floats rather than being biased as expected.
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Drive is emulated by switching the line to an input when the line should not
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be driven.
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In all cases, the configuration reported by gpio-get-lineinfo-ioctl.rst
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is the requested configuration, not the resulting hardware configuration.
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Userspace cannot determine if a feature is supported in hardware, is
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emulated, or is best effort.
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Return Value
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============
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On success 0 and the :c:type:`request.fd<gpiohandle_request>` contains the
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file descriptor for the request.
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On error -1 and the ``errno`` variable is set appropriately.
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Common error codes are described in error-codes.rst.
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