1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================= 4 Industrial IIO device buffers 5 ============================= 6 7 1. Overview 8 =========== 9 10 The Industrial I/O core offers a way for continuous data capture based on a 11 trigger source. Multiple data channels can be read at once from 12 ``/dev/iio:deviceX`` character device node, thus reducing the CPU load. 13 14 Devices with buffer support feature an additional sub-directory in the 15 ``/sys/bus/iio/devices/iio:deviceX/`` directory hierarchy, called bufferY, where 16 Y defaults to 0, for devices with a single buffer. 17 18 2. Buffer attributes 19 ==================== 20 21 An IIO buffer has an associated attributes directory under 22 ``/sys/bus/iio/iio:deviceX/bufferY/``. The attributes are described below. 23 24 ``length`` 25 ---------- 26 27 Read / Write attribute which states the total number of data samples (capacity) 28 that can be stored by the buffer. 29 30 ``enable`` 31 ---------- 32 33 Read / Write attribute which starts / stops the buffer capture. This file should 34 be written last, after length and selection of scan elements. Writing a non-zero 35 value may result in an error, such as EINVAL, if, for example, an unsupported 36 combination of channels is given. 37 38 ``watermark`` 39 ------------- 40 41 Read / Write positive integer attribute specifying the maximum number of scan 42 elements to wait for. 43 44 Poll will block until the watermark is reached. 45 46 Blocking read will wait until the minimum between the requested read amount or 47 the low watermark is available. 48 49 Non-blocking read will retrieve the available samples from the buffer even if 50 there are less samples than the watermark level. This allows the application to 51 block on poll with a timeout and read the available samples after the timeout 52 expires and thus have a maximum delay guarantee. 53 54 Data available 55 -------------- 56 57 Read-only attribute indicating the bytes of data available in the buffer. In the 58 case of an output buffer, this indicates the amount of empty space available to 59 write data to. In the case of an input buffer, this indicates the amount of data 60 available for reading. 61 62 Scan elements 63 ------------- 64 65 The meta information associated with a channel data placed in a buffer is called 66 a scan element. The scan elements attributes are presented below. 67 68 **_en** 69 70 Read / Write attribute used for enabling a channel. If and only if its value 71 is non-zero, then a triggered capture will contain data samples for this 72 channel. 73 74 **_index** 75 76 Read-only unsigned integer attribute specifying the position of the channel in 77 the buffer. Note these are not dependent on what is enabled and may not be 78 contiguous. Thus for userspace to establish the full layout these must be used 79 in conjunction with all _en attributes to establish which channels are present, 80 and the relevant _type attributes to establish the data storage format. 81 82 **_type** 83 84 Read-only attribute containing the description of the scan element data storage 85 within the buffer and hence the form in which it is read from userspace. Format 86 is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift], where: 87 88 - **be** or **le** specifies big or little-endian. 89 - **s** or **u** specifies if signed (2's complement) or unsigned. 90 - **bits** is the number of valid data bits. 91 - **storagebits** is the number of bits (after padding) that it occupies in the 92 buffer. 93 - **repeat** specifies the number of bits/storagebits repetitions. When the 94 repeat element is 0 or 1, then the repeat value is omitted. 95 - **shift** if specified, is the shift that needs to be applied prior to 96 masking out unused bits. 97 98 For example, a driver for a 3-axis accelerometer with 12-bit resolution where 99 data is stored in two 8-bit registers is as follows:: 100 101 7 6 5 4 3 2 1 0 102 +---+---+---+---+---+---+---+---+ 103 |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06) 104 +---+---+---+---+---+---+---+---+ 105 106 7 6 5 4 3 2 1 0 107 +---+---+---+---+---+---+---+---+ 108 |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07) 109 +---+---+---+---+---+---+---+---+ 110 111 will have the following scan element type for each axis: 112 113 .. code-block:: bash 114 115 $ cat /sys/bus/iio/devices/iio:device0/buffer0/in_accel_y_type 116 le:s12/16>>4 117 118 A userspace application will interpret data samples read from the buffer as 119 two-byte little-endian signed data, that needs a 4 bits right shift before 120 masking out the 12 valid bits of data. 121 122 It is also worth mentioning that the data in the buffer will be naturally 123 aligned, so the userspace application has to handle the buffers accordingly. 124 125 Take for example, a driver with four channels with the following description: 126 - channel0: index: 0, type: be:u16/16>>0 127 - channel1: index: 1, type: be:u32/32>>0 128 - channel2: index: 2, type: be:u32/32>>0 129 - channel3: index: 3, type: be:u64/64>>0 130 131 If all channels are enabled, the data will be aligned in the buffer as follows:: 132 133 0-1 2 3 4-7 8-11 12 13 14 15 16-23 -> buffer byte number 134 +-----+---+---+-----+-----+---+---+---+---+-----+ 135 |CHN_0|PAD|PAD|CHN_1|CHN_2|PAD|PAD|PAD|PAD|CHN_3| -> buffer content 136 +-----+---+---+-----+-----+---+---+---+---+-----+ 137 138 If only channel0 and channel3 are enabled, the data will be aligned in the 139 buffer as follows:: 140 141 0-1 2 3 4 5 6 7 8-15 -> buffer byte number 142 +-----+---+---+---+---+---+---+-----+ 143 |CHN_0|PAD|PAD|PAD|PAD|PAD|PAD|CHN_3| -> buffer content 144 +-----+---+---+---+---+---+---+-----+ 145 146 Typically the buffered data is found in raw format (unscaled with no offset 147 applied), however there are corner cases in which the buffered data may be found 148 in a processed form. Please note that these corner cases are not addressed by 149 this documentation. 150 151 Please see ``Documentation/ABI/testing/sysfs-bus-iio`` for a complete 152 description of the attributes.
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