1 .. SPDX-License-Identifier: GPL-2.0
2 
3 .. _GPIO_V2_GET_LINE_IOCTL:
4 
5 **********************
6 GPIO_V2_GET_LINE_IOCTL
7 **********************
8 
9 Name
10 ====
11 
12 GPIO_V2_GET_LINE_IOCTL - Request a line or lines from the kernel.
13 
14 Synopsis
15 ========
16 
17 .. c:macro:: GPIO_V2_GET_LINE_IOCTL
18 
19 ``int ioctl(int chip_fd, GPIO_V2_GET_LINE_IOCTL, struct gpio_v2_line_request *request)``
20 
21 Arguments
22 =========
23 
24 ``chip_fd``
25     The file descriptor of the GPIO character device returned by `open()`.
26 
27 ``request``
28     The :c:type:`line_request<gpio_v2_line_request>` specifying the lines
29     to request and their configuration.
30 
31 Description
32 ===========
33 
34 On success, the requesting process is granted exclusive access to the line
35 value, write access to the line configuration, and may receive events when
36 edges are detected on the line, all of which are described in more detail in
37 :ref:`gpio-v2-line-request`.
38 
39 A number of lines may be requested in the one line request, and request
40 operations are performed on the requested lines by the kernel as atomically
41 as possible. e.g. gpio-v2-line-get-values-ioctl.rst will read all the
42 requested lines at once.
43 
44 The state of a line, including the value of output lines, is guaranteed to
45 remain as requested until the returned file descriptor is closed. Once the
46 file descriptor is closed, the state of the line becomes uncontrolled from
47 the userspace perspective, and may revert to its default state.
48 
49 Requesting a line already in use is an error (**EBUSY**).
50 
51 Closing the ``chip_fd`` has no effect on existing line requests.
52 
53 .. _gpio-v2-get-line-config-rules:
54 
55 Configuration Rules
56 -------------------
57 
58 For any given requested line, the following configuration rules apply:
59 
60 The direction flags, ``GPIO_V2_LINE_FLAG_INPUT`` and
61 ``GPIO_V2_LINE_FLAG_OUTPUT``, cannot be combined. If neither are set then
62 the only other flag that may be set is ``GPIO_V2_LINE_FLAG_ACTIVE_LOW``
63 and the line is requested "as-is" to allow reading of the line value
64 without altering the electrical configuration.
65 
66 The drive flags, ``GPIO_V2_LINE_FLAG_OPEN_xxx``, require the
67 ``GPIO_V2_LINE_FLAG_OUTPUT`` to be set.
68 Only one drive flag may be set.
69 If none are set then the line is assumed push-pull.
70 
71 Only one bias flag, ``GPIO_V2_LINE_FLAG_BIAS_xxx``, may be set, and it
72 requires a direction flag to also be set.
73 If no bias flags are set then the bias configuration is not changed.
74 
75 The edge flags, ``GPIO_V2_LINE_FLAG_EDGE_xxx``, require
76 ``GPIO_V2_LINE_FLAG_INPUT`` to be set and may be combined to detect both rising
77 and falling edges.  Requesting edge detection from a line that does not support
78 it is an error (**ENXIO**).
79 
80 Only one event clock flag, ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_xxx``, may be set.
81 If none are set then the event clock defaults to ``CLOCK_MONOTONIC``.
82 The ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE`` flag requires supporting hardware
83 and a kernel with ``CONFIG_HTE`` set.  Requesting HTE from a device that
84 doesn't support it is an error (**EOPNOTSUPP**).
85 
86 The :c:type:`debounce_period_us<gpio_v2_line_attribute>` attribute may only
87 be applied to lines with ``GPIO_V2_LINE_FLAG_INPUT`` set. When set, debounce
88 applies to both the values returned by gpio-v2-line-get-values-ioctl.rst and
89 the edges returned by gpio-v2-line-event-read.rst.  If not
90 supported directly by hardware, debouncing is emulated in software by the
91 kernel.  Requesting debounce on a line that supports neither debounce in
92 hardware nor interrupts, as required for software emulation, is an error
93 (**ENXIO**).
94 
95 Requesting an invalid configuration is an error (**EINVAL**).
96 
97 .. _gpio-v2-get-line-config-support:
98 
99 Configuration Support
100 ---------------------
101 
102 Where the requested configuration is not directly supported by the underlying
103 hardware and driver, the kernel applies one of these approaches:
104 
105  - reject the request
106  - emulate the feature in software
107  - treat the feature as best effort
108 
109 The approach applied depends on whether the feature can reasonably be emulated
110 in software, and the impact on the hardware and userspace if the feature is not
111 supported.
112 The approach applied for each feature is as follows:
113 
114 ==============   ===========
115 Feature          Approach
116 ==============   ===========
117 Bias             best effort
118 Debounce         emulate
119 Direction        reject
120 Drive            emulate
121 Edge Detection   reject
122 ==============   ===========
123 
124 Bias is treated as best effort to allow userspace to apply the same
125 configuration for platforms that support internal bias as those that require
126 external bias.
127 Worst case the line floats rather than being biased as expected.
128 
129 Debounce is emulated by applying a filter to hardware interrupts on the line.
130 An edge event is generated after an edge is detected and the line remains
131 stable for the debounce period.
132 The event timestamp corresponds to the end of the debounce period.
133 
134 Drive is emulated by switching the line to an input when the line should not
135 be actively driven.
136 
137 Edge detection requires interrupt support, and is rejected if that is not
138 supported. Emulation by polling can still be performed from userspace.
139 
140 In all cases, the configuration reported by gpio-v2-get-lineinfo-ioctl.rst
141 is the requested configuration, not the resulting hardware configuration.
142 Userspace cannot determine if a feature is supported in hardware, is
143 emulated, or is best effort.
144 
145 Return Value
146 ============
147 
148 On success 0 and the :c:type:`request.fd<gpio_v2_line_request>` contains the
149 file descriptor for the request.
150 
151 On error -1 and the ``errno`` variable is set appropriately.
152 Common error codes are described in error-codes.rst.
153