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1.. SPDX-License-Identifier: GPL-2.02 3.. _GPIO_V2_LINEINFO_CHANGED_READ:4 5*****************************6GPIO_V2_LINEINFO_CHANGED_READ7*****************************8 9Name10====11 12GPIO_V2_LINEINFO_CHANGED_READ - Read line info changed events for watched13lines from the chip.14 15Synopsis16========17 18``int read(int chip_fd, void *buf, size_t count)``19 20Arguments21=========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 size31 of a :c:type:`gpio_v2_line_info_changed` event.32 33Description34===========35 36Read line info changed events for watched lines from the chip.37 38.. note::39 Monitoring line info changes is not generally required, and would typically40 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 a44 line changes value.45 46A line must be watched using gpio-v2-get-lineinfo-watch-ioctl.rst to generate47info changed events. Subsequently, a request, release, or reconfiguration48of the line will generate an info changed event.49 50The kernel timestamps events when they occur and stores them in a buffer51from where they can be read by userspace at its convenience using `read()`.52 53The size of the kernel event buffer is fixed at 32 events per ``chip_fd``.54 55The buffer may overflow if bursts of events occur quicker than they are read56by userspace. If an overflow occurs then the most recent event is discarded.57Overflow cannot be detected from userspace.58 59Events read from the buffer are always in the same order that they were60detected by the kernel, including when multiple lines are being monitored by61the one ``chip_fd``.62 63To minimize the number of calls required to copy events from the kernel to64userspace, `read()` supports copying multiple events. The number of events65copied is the lower of the number available in the kernel buffer and the66number that will fit in the userspace buffer (``buf``).67 68A `read()` will block if no event is available and the ``chip_fd`` has not69been set **O_NONBLOCK**.70 71The presence of an event can be tested for by checking that the ``chip_fd`` is72readable using `poll()` or an equivalent.73 74Return Value75============76 77On success the number of bytes read, which will be a multiple of the size78of a :c:type:`gpio_v2_line_info_changed` event.79 80On error -1 and the ``errno`` variable is set appropriately.81Common error codes are described in error-codes.rst.82