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Diffstat (limited to 'Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst')
-rw-r--r-- | Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst | 83 |
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diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst b/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst new file mode 100644 index 000000000000..6513c23fb7ca --- /dev/null +++ b/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst @@ -0,0 +1,83 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _GPIO_V2_LINE_EVENT_READ: + +*********************** +GPIO_V2_LINE_EVENT_READ +*********************** + +Name +==== + +GPIO_V2_LINE_EVENT_READ - Read edge detection events for lines from a request. + +Synopsis +======== + +``int read(int req_fd, void *buf, size_t count)`` + +Arguments +========= + +``req_fd`` + The file descriptor of the GPIO character device, as returned in the + :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst. + +``buf`` + The buffer to contain the :c:type:`events<gpio_v2_line_event>`. + +``count`` + The number of bytes available in ``buf``, which must be at + least the size of a :c:type:`gpio_v2_line_event`. + +Description +=========== + +Read edge detection events for lines from a request. + +Edge detection must be enabled for the input line using either +``GPIO_V2_LINE_FLAG_EDGE_RISING`` or ``GPIO_V2_LINE_FLAG_EDGE_FALLING``, or +both. Edge events are then generated whenever edge interrupts are detected on +the input line. + +The kernel captures and timestamps edge events as close as possible to their +occurrence and stores them in a buffer from where they can be read by +userspace at its convenience using `read()`. + +Events read from the buffer are always in the same order that they were +detected by the kernel, including when multiple lines are being monitored by +the one request. + +The size of the kernel event buffer is fixed at the time of line request +creation, and can be influenced by the +:c:type:`request.event_buffer_size<gpio_v2_line_request>`. +The default size is 16 times the number of lines requested. + +The buffer may overflow if bursts of events occur quicker than they are read +by userspace. If an overflow occurs then the oldest buffered event is +discarded. Overflow can be detected from userspace by monitoring the event +sequence numbers. + +To minimize the number of calls required to copy events from the kernel to +userspace, `read()` supports copying multiple events. The number of events +copied is the lower of the number available in the kernel buffer and the +number that will fit in the userspace buffer (``buf``). + +Changing the edge detection flags using gpio-v2-line-set-config-ioctl.rst +does not remove or modify the events already contained in the kernel event +buffer. + +The `read()` will block if no event is available and the ``req_fd`` has not +been set **O_NONBLOCK**. + +The presence of an event can be tested for by checking that the ``req_fd`` is +readable using `poll()` or an equivalent. + +Return Value +============ + +On success the number of bytes read, which will be a multiple of the size of a +:c:type:`gpio_v2_line_event` event. + +On error -1 and the ``errno`` variable is set appropriately. +Common error codes are described in error-codes.rst. |