1 .. SPDX-License-Identifier: GPL-2.0
2
3 .. _GPIO_V2_LINE_EVENT_READ:
4
5 ***********************
6 GPIO_V2_LINE_EVENT_READ
7 ***********************
8
9 Name
10 ====
11
12 GPIO_V2_LINE_EVENT_READ - Read edge detection
13
14 Synopsis
15 ========
16
17 ``int read(int req_fd, void *buf, size_t count
18
19 Arguments
20 =========
21
22 ``req_fd``
23 The file descriptor of the GPIO character
24 :c:type:`request.fd<gpio_v2_line_request>`
25
26 ``buf``
27 The buffer to contain the :c:type:`events<
28
29 ``count``
30 The number of bytes available in ``buf``,
31 least the size of a :c:type:`gpio_v2_line_
32
33 Description
34 ===========
35
36 Read edge detection events for lines from a re
37
38 Edge detection must be enabled for the input l
39 ``GPIO_V2_LINE_FLAG_EDGE_RISING`` or ``GPIO_V2
40 both. Edge events are then generated whenever
41 the input line.
42
43 Edges are defined in terms of changes to the l
44 to active transition is a rising edge. If ``G
45 set then logical polarity is the opposite of p
46 ``GPIO_V2_LINE_FLAG_EDGE_RISING`` then corresp
47
48 The kernel captures and timestamps edge events
49 occurrence and stores them in a buffer from wh
50 userspace at its convenience using `read()`.
51
52 Events read from the buffer are always in the
53 detected by the kernel, including when multipl
54 the one request.
55
56 The size of the kernel event buffer is fixed a
57 creation, and can be influenced by the
58 :c:type:`request.event_buffer_size<gpio_v2_lin
59 The default size is 16 times the number of lin
60
61 The buffer may overflow if bursts of events oc
62 by userspace. If an overflow occurs then the o
63 discarded. Overflow can be detected from users
64 sequence numbers.
65
66 To minimize the number of calls required to co
67 userspace, `read()` supports copying multiple
68 copied is the lower of the number available in
69 number that will fit in the userspace buffer (
70
71 Changing the edge detection flags using gpio-v
72 does not remove or modify the events already c
73 buffer.
74
75 The `read()` will block if no event is availab
76 been set **O_NONBLOCK**.
77
78 The presence of an event can be tested for by
79 readable using `poll()` or an equivalent.
80
81 Return Value
82 ============
83
84 On success the number of bytes read, which wil
85 :c:type:`gpio_v2_line_event` event.
86
87 On error -1 and the ``errno`` variable is set
88 Common error codes are described in error-code
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.