1 .. SPDX-License-Identifier: GPL-2.0
3 .. _GPIO_V2_LINE_EVENT_READ:
5 ***********************
6 GPIO_V2_LINE_EVENT_READ
7 ***********************
12 GPIO_V2_LINE_EVENT_READ - Read edge detection events for lines from a request.
17 ``int read(int req_fd, void *buf, size_t count)``
23 The file descriptor of the GPIO character device, as returned in the
24 :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
27 The buffer to contain the :c:type:`events<gpio_v2_line_event>`.
30 The number of bytes available in ``buf``, which must be at
31 least the size of a :c:type:`gpio_v2_line_event`.
36 Read edge detection events for lines from a request.
38 Edge detection must be enabled for the input line using either
39 ``GPIO_V2_LINE_FLAG_EDGE_RISING`` or ``GPIO_V2_LINE_FLAG_EDGE_FALLING``, or
40 both. Edge events are then generated whenever edge interrupts are detected on
43 Edges are defined in terms of changes to the logical line value, so an inactive
44 to active transition is a rising edge. If ``GPIO_V2_LINE_FLAG_ACTIVE_LOW`` is
45 set then logical polarity is the opposite of physical polarity, and
46 ``GPIO_V2_LINE_FLAG_EDGE_RISING`` then corresponds to a falling physical edge.
48 The kernel captures and timestamps edge events as close as possible to their
49 occurrence and stores them in a buffer from where they can be read by
50 userspace at its convenience using `read()`.
52 Events read from the buffer are always in the same order that they were
53 detected by the kernel, including when multiple lines are being monitored by
56 The size of the kernel event buffer is fixed at the time of line request
57 creation, and can be influenced by the
58 :c:type:`request.event_buffer_size<gpio_v2_line_request>`.
59 The default size is 16 times the number of lines requested.
61 The buffer may overflow if bursts of events occur quicker than they are read
62 by userspace. If an overflow occurs then the oldest buffered event is
63 discarded. Overflow can be detected from userspace by monitoring the event
66 To minimize the number of calls required to copy events from the kernel to
67 userspace, `read()` supports copying multiple events. The number of events
68 copied is the lower of the number available in the kernel buffer and the
69 number that will fit in the userspace buffer (``buf``).
71 Changing the edge detection flags using gpio-v2-line-set-config-ioctl.rst
72 does not remove or modify the events already contained in the kernel event
75 The `read()` will block if no event is available and the ``req_fd`` has not
76 been set **O_NONBLOCK**.
78 The presence of an event can be tested for by checking that the ``req_fd`` is
79 readable using `poll()` or an equivalent.
84 On success the number of bytes read, which will be a multiple of the size of a
85 :c:type:`gpio_v2_line_event` event.
87 On error -1 and the ``errno`` variable is set appropriately.
88 Common error codes are described in error-codes.rst.
projects / thirdparty / kernel / linux.git / blob