85 lines
2.7 KiB
ReStructuredText
85 lines
2.7 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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.. _GPIO_LINEEVENT_DATA_READ:
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************************
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GPIO_LINEEVENT_DATA_READ
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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-line-event-read.rst.
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Name
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====
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GPIO_LINEEVENT_DATA_READ - Read edge detection events from a line event.
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Synopsis
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========
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``int read(int event_fd, void *buf, size_t count)``
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Arguments
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=========
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``event_fd``
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The file descriptor of the GPIO character device, as returned in the
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:c:type:`request.fd<gpioevent_request>` by gpio-get-lineevent-ioctl.rst.
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``buf``
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The buffer to contain the :c:type:`events<gpioevent_data>`.
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``count``
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The number of bytes available in ``buf``, which must be at
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least the size of a :c:type:`gpioevent_data`.
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Description
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===========
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Read edge detection events for a line from a line event.
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Edge detection must be enabled for the input line using either
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``GPIOEVENT_REQUEST_RISING_EDGE`` or ``GPIOEVENT_REQUEST_FALLING_EDGE``, or
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both. Edge events are then generated whenever edge interrupts are detected on
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the input line.
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The kernel captures and timestamps edge events as close as possible to their
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occurrence and stores them in a buffer from where they can be read by
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userspace at its convenience using `read()`.
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The source of the clock for :c:type:`event.timestamp<gpioevent_data>` is
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``CLOCK_MONOTONIC``, except for kernels earlier than Linux 5.7 when it was
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``CLOCK_REALTIME``. There is no indication in the :c:type:`gpioevent_data`
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as to which clock source is used, it must be determined from either the kernel
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version or sanity checks on the timestamp itself.
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Events read from the buffer are always in the same order that they were
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detected by the kernel.
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The size of the kernel event buffer is fixed at 16 events.
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The buffer may overflow if bursts of events occur quicker than they are read
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by userspace. If an overflow occurs then the most recent event is discarded.
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Overflow cannot be detected from userspace.
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To minimize the number of calls required to copy events from the kernel to
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userspace, `read()` supports copying multiple events. The number of events
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copied is the lower of the number available in the kernel buffer and the
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number that will fit in the userspace buffer (``buf``).
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The `read()` will block if no event is available and the ``event_fd`` has not
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been set **O_NONBLOCK**.
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The presence of an event can be tested for by checking that the ``event_fd`` is
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readable using `poll()` or an equivalent.
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Return Value
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============
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On success the number of bytes read, which will be a multiple of the size of
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a :c:type:`gpio_lineevent_data` event.
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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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