a261e208e1
EOPNOTSUPP has two 'P's. Signed-off-by: David Lechner <dlechner@baylibre.com> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
153 lines
5.4 KiB
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153 lines
5.4 KiB
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.. SPDX-License-Identifier: GPL-2.0
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.. _GPIO_V2_GET_LINE_IOCTL:
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**********************
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GPIO_V2_GET_LINE_IOCTL
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**********************
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Name
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====
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GPIO_V2_GET_LINE_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_V2_GET_LINE_IOCTL
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``int ioctl(int chip_fd, GPIO_V2_GET_LINE_IOCTL, struct gpio_v2_line_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:`line_request<gpio_v2_line_request>` specifying the lines
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to request and their configuration.
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Description
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===========
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On success, the requesting process is granted exclusive access to the line
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value, write access to the line configuration, and may receive events when
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edges are detected on the line, all of which are described in more detail in
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:ref:`gpio-v2-line-request`.
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A number of lines may be requested in the one line request, and request
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operations are performed on the requested lines by the kernel as atomically
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as possible. e.g. gpio-v2-line-get-values-ioctl.rst will read all the
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requested lines at once.
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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 requests.
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.. _gpio-v2-get-line-config-rules:
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Configuration Rules
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-------------------
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For any given requested line, the following configuration rules apply:
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The direction flags, ``GPIO_V2_LINE_FLAG_INPUT`` and
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``GPIO_V2_LINE_FLAG_OUTPUT``, cannot be combined. If neither are set then
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the only other flag that may be set is ``GPIO_V2_LINE_FLAG_ACTIVE_LOW``
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and the line is requested "as-is" to allow reading of the line value
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without altering the electrical configuration.
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The drive flags, ``GPIO_V2_LINE_FLAG_OPEN_xxx``, require the
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``GPIO_V2_LINE_FLAG_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, ``GPIO_V2_LINE_FLAG_BIAS_xxx``, may be set, and it
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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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The edge flags, ``GPIO_V2_LINE_FLAG_EDGE_xxx``, require
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``GPIO_V2_LINE_FLAG_INPUT`` to be set and may be combined to detect both rising
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and falling edges. Requesting edge detection from a line that does not support
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it is an error (**ENXIO**).
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Only one event clock flag, ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_xxx``, may be set.
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If none are set then the event clock defaults to ``CLOCK_MONOTONIC``.
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The ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE`` flag requires supporting hardware
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and a kernel with ``CONFIG_HTE`` set. Requesting HTE from a device that
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doesn't support it is an error (**EOPNOTSUPP**).
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The :c:type:`debounce_period_us<gpio_v2_line_attribute>` attribute may only
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be applied to lines with ``GPIO_V2_LINE_FLAG_INPUT`` set. When set, debounce
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applies to both the values returned by gpio-v2-line-get-values-ioctl.rst and
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the edges returned by gpio-v2-line-event-read.rst. If not
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supported directly by hardware, debouncing is emulated in software by the
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kernel. Requesting debounce on a line that supports neither debounce in
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hardware nor interrupts, as required for software emulation, is an error
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(**ENXIO**).
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Requesting an invalid configuration is an error (**EINVAL**).
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.. _gpio-v2-get-line-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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Debounce emulate
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Direction reject
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Drive emulate
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Edge Detection reject
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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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Debounce is emulated by applying a filter to hardware interrupts on the line.
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An edge event is generated after an edge is detected and the line remains
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stable for the debounce period.
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The event timestamp corresponds to the end of the debounce period.
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Drive is emulated by switching the line to an input when the line should not
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be actively driven.
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Edge detection requires interrupt support, and is rejected if that is not
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supported. Emulation by polling can still be performed from userspace.
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In all cases, the configuration reported by gpio-v2-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<gpio_v2_line_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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