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 events for lines from a request. 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 device, as returned in the 24 :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst. 25 26 ``buf`` 27 The buffer to contain the :c:type:`events<gpio_v2_line_event>`. 28 29 ``count`` 30 The number of bytes available in ``buf``, which must be at 31 least the size of a :c:type:`gpio_v2_line_event`. 32 33 Description 34 =========== 35 36 Read edge detection events for lines from a request. 37 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 41 the input line. 42 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. 47 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()`. 51 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 54 the one request. 55 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. 60 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 64 sequence numbers. 65 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``). 70 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 73 buffer. 74 75 The `read()` will block if no event is available and the ``req_fd`` has not 76 been set **O_NONBLOCK**. 77 78 The presence of an event can be tested for by checking that the ``req_fd`` is 79 readable using `poll()` or an equivalent. 80 81 Return Value 82 ============ 83 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. 86 87 On error -1 and the ``errno`` variable is set appropriately. 88 Common error codes are described in error-codes.rst.
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