1 .. SPDX-License-Identifier: GPL-2.0
2
3 .. _GPIO_LINEEVENT_DATA_READ:
4
5 ************************
6 GPIO_LINEEVENT_DATA_READ
7 ************************
8
9 .. warning::
10 This ioctl is part of chardev_v1.rst and i
11 gpio-v2-line-event-read.rst.
12
13 Name
14 ====
15
16 GPIO_LINEEVENT_DATA_READ - Read edge detection
17
18 Synopsis
19 ========
20
21 ``int read(int event_fd, void *buf, size_t cou
22
23 Arguments
24 =========
25
26 ``event_fd``
27 The file descriptor of the GPIO character
28 :c:type:`request.fd<gpioevent_request>` by
29
30 ``buf``
31 The buffer to contain the :c:type:`events<
32
33 ``count``
34 The number of bytes available in ``buf``,
35 least the size of a :c:type:`gpioevent_dat
36
37 Description
38 ===========
39
40 Read edge detection events for a line from a l
41
42 Edge detection must be enabled for the input l
43 ``GPIOEVENT_REQUEST_RISING_EDGE`` or ``GPIOEVE
44 both. Edge events are then generated whenever
45 the input line.
46
47 Edges are defined in terms of changes to the l
48 to active transition is a rising edge. If ``G
49 set then logical polarity is the opposite of p
50 ``GPIOEVENT_REQUEST_RISING_EDGE`` then corresp
51
52 The kernel captures and timestamps edge events
53 occurrence and stores them in a buffer from wh
54 userspace at its convenience using `read()`.
55
56 The source of the clock for :c:type:`event.tim
57 ``CLOCK_MONOTONIC``, except for kernels earlie
58 ``CLOCK_REALTIME``. There is no indication in
59 as to which clock source is used, it must be d
60 version or sanity checks on the timestamp itse
61
62 Events read from the buffer are always in the
63 detected by the kernel.
64
65 The size of the kernel event buffer is fixed a
66
67 The buffer may overflow if bursts of events oc
68 by userspace. If an overflow occurs then the m
69 Overflow cannot be detected from userspace.
70
71 To minimize the number of calls required to co
72 userspace, `read()` supports copying multiple
73 copied is the lower of the number available in
74 number that will fit in the userspace buffer (
75
76 The `read()` will block if no event is availab
77 been set **O_NONBLOCK**.
78
79 The presence of an event can be tested for by
80 readable using `poll()` or an equivalent.
81
82 Return Value
83 ============
84
85 On success the number of bytes read, which wil
86 a :c:type:`gpio_lineevent_data` event.
87
88 On error -1 and the ``errno`` variable is set
89 Common error codes are described in error-code
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