1 .. SPDX-License-Identifier: GPL-2.0 2 3 .. _GPIO_V2_LINEINFO_CHANGED_READ: 4 5 ***************************** 6 GPIO_V2_LINEINFO_CHANGED_READ 7 ***************************** 8 9 Name 10 ==== 11 12 GPIO_V2_LINEINFO_CHANGED_READ - Read line info changed events for watched 13 lines from the chip. 14 15 Synopsis 16 ======== 17 18 ``int read(int chip_fd, void *buf, size_t count)`` 19 20 Arguments 21 ========= 22 23 ``chip_fd`` 24 The file descriptor of the GPIO character device returned by `open()`. 25 26 ``buf`` 27 The buffer to contain the :c:type:`events<gpio_v2_line_info_changed>`. 28 29 ``count`` 30 The number of bytes available in ``buf``, which must be at least the size 31 of a :c:type:`gpio_v2_line_info_changed` event. 32 33 Description 34 =========== 35 36 Read line info changed events for watched lines from the chip. 37 38 .. note:: 39 Monitoring line info changes is not generally required, and would typically 40 only be performed by a system monitoring component. 41 42 These events relate to changes in a line's request state or configuration, 43 not its value. Use gpio-v2-line-event-read.rst to receive events when a 44 line changes value. 45 46 A line must be watched using gpio-v2-get-lineinfo-watch-ioctl.rst to generate 47 info changed events. Subsequently, a request, release, or reconfiguration 48 of the line will generate an info changed event. 49 50 The kernel timestamps events when they occur and stores them in a buffer 51 from where they can be read by userspace at its convenience using `read()`. 52 53 The size of the kernel event buffer is fixed at 32 events per ``chip_fd``. 54 55 The buffer may overflow if bursts of events occur quicker than they are read 56 by userspace. If an overflow occurs then the most recent event is discarded. 57 Overflow cannot be detected from userspace. 58 59 Events read from the buffer are always in the same order that they were 60 detected by the kernel, including when multiple lines are being monitored by 61 the one ``chip_fd``. 62 63 To minimize the number of calls required to copy events from the kernel to 64 userspace, `read()` supports copying multiple events. The number of events 65 copied is the lower of the number available in the kernel buffer and the 66 number that will fit in the userspace buffer (``buf``). 67 68 A `read()` will block if no event is available and the ``chip_fd`` has not 69 been set **O_NONBLOCK**. 70 71 The presence of an event can be tested for by checking that the ``chip_fd`` is 72 readable using `poll()` or an equivalent. 73 74 Return Value 75 ============ 76 77 On success the number of bytes read, which will be a multiple of the size 78 of a :c:type:`gpio_v2_line_info_changed` event. 79 80 On error -1 and the ``errno`` variable is set appropriately. 81 Common error codes are described in error-codes.rst.
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