1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 .. _GPIO_LINEINFO_CHANGED_READ: 3 .. _GPIO_LINEINFO_CHANGED_READ:
4 4
5 ************************** 5 **************************
6 GPIO_LINEINFO_CHANGED_READ 6 GPIO_LINEINFO_CHANGED_READ
7 ************************** 7 **************************
8 8
9 .. warning:: 9 .. warning::
10 This ioctl is part of chardev_v1.rst and i 10 This ioctl is part of chardev_v1.rst and is obsoleted by
11 gpio-v2-lineinfo-changed-read.rst. 11 gpio-v2-lineinfo-changed-read.rst.
12 12
13 Name 13 Name
14 ==== 14 ====
15 15
16 GPIO_LINEINFO_CHANGED_READ - Read line info ch 16 GPIO_LINEINFO_CHANGED_READ - Read line info change events for watched lines
17 from the chip. 17 from the chip.
18 18
19 Synopsis 19 Synopsis
20 ======== 20 ========
21 21
22 ``int read(int chip_fd, void *buf, size_t coun 22 ``int read(int chip_fd, void *buf, size_t count)``
23 23
24 Arguments 24 Arguments
25 ========= 25 =========
26 26
27 ``chip_fd`` 27 ``chip_fd``
28 The file descriptor of the GPIO character 28 The file descriptor of the GPIO character device returned by `open()`.
29 29
30 ``buf`` 30 ``buf``
31 The buffer to contain the :c:type:`events< 31 The buffer to contain the :c:type:`events<gpioline_info_changed>`.
32 32
33 ``count`` 33 ``count``
34 The number of bytes available in ``buf``, 34 The number of bytes available in ``buf``, which must be at least the size
35 of a :c:type:`gpioline_info_changed` event 35 of a :c:type:`gpioline_info_changed` event.
36 36
37 Description 37 Description
38 =========== 38 ===========
39 39
40 Read line info change events for watched lines 40 Read line info change events for watched lines from the chip.
41 41
42 .. note:: 42 .. note::
43 Monitoring line info changes is not genera 43 Monitoring line info changes is not generally required, and would typically
44 only be performed by a system monitoring c 44 only be performed by a system monitoring component.
45 45
46 These events relate to changes in a line's 46 These events relate to changes in a line's request state or configuration,
47 not its value. Use gpio-lineevent-data-rea 47 not its value. Use gpio-lineevent-data-read.rst to receive events when a
48 line changes value. 48 line changes value.
49 49
50 A line must be watched using gpio-get-lineinfo 50 A line must be watched using gpio-get-lineinfo-watch-ioctl.rst to generate
51 info changed events. Subsequently, a request, 51 info changed events. Subsequently, a request, release, or reconfiguration
52 of the line will generate an info changed even 52 of the line will generate an info changed event.
53 53
54 The kernel timestamps events when they occur a 54 The kernel timestamps events when they occur and stores them in a buffer
55 from where they can be read by userspace at it 55 from where they can be read by userspace at its convenience using `read()`.
56 56
57 The size of the kernel event buffer is fixed a 57 The size of the kernel event buffer is fixed at 32 events per ``chip_fd``.
58 58
59 The buffer may overflow if bursts of events oc 59 The buffer may overflow if bursts of events occur quicker than they are read
60 by userspace. If an overflow occurs then the m 60 by userspace. If an overflow occurs then the most recent event is discarded.
61 Overflow cannot be detected from userspace. 61 Overflow cannot be detected from userspace.
62 62
63 Events read from the buffer are always in the 63 Events read from the buffer are always in the same order that they were
64 detected by the kernel, including when multipl 64 detected by the kernel, including when multiple lines are being monitored by
65 the one ``chip_fd``. 65 the one ``chip_fd``.
66 66
67 To minimize the number of calls required to co 67 To minimize the number of calls required to copy events from the kernel to
68 userspace, `read()` supports copying multiple 68 userspace, `read()` supports copying multiple events. The number of events
69 copied is the lower of the number available in 69 copied is the lower of the number available in the kernel buffer and the
70 number that will fit in the userspace buffer ( 70 number that will fit in the userspace buffer (``buf``).
71 71
72 A `read()` will block if no event is available 72 A `read()` will block if no event is available and the ``chip_fd`` has not
73 been set **O_NONBLOCK**. 73 been set **O_NONBLOCK**.
74 74
75 The presence of an event can be tested for by 75 The presence of an event can be tested for by checking that the ``chip_fd`` is
76 readable using `poll()` or an equivalent. 76 readable using `poll()` or an equivalent.
77 77
78 First added in 5.7. 78 First added in 5.7.
79 79
80 Return Value 80 Return Value
81 ============ 81 ============
82 82
83 On success the number of bytes read, which wil 83 On success the number of bytes read, which will be a multiple of the size of
84 a :c:type:`gpioline_info_changed` event. 84 a :c:type:`gpioline_info_changed` event.
85 85
86 On error -1 and the ``errno`` variable is set 86 On error -1 and the ``errno`` variable is set appropriately.
87 Common error codes are described in error-code 87 Common error codes are described in error-codes.rst.
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