1======= 2Buffers 3======= 4 5* struct iio_buffer — general buffer structure 6* :c:func:`iio_validate_scan_mask_onehot` — Validates that exactly one channel 7 is selected 8* :c:func:`iio_buffer_get` — Grab a reference to the buffer 9* :c:func:`iio_buffer_put` — Release the reference to the buffer 10 11The Industrial I/O core offers a way for continuous data capture based on a 12trigger source. Multiple data channels can be read at once from 13:file:`/dev/iio:device{X}` character device node, thus reducing the CPU load. 14 15IIO buffer sysfs interface 16========================== 17An IIO buffer has an associated attributes directory under 18:file:`/sys/bus/iio/iio:device{X}/buffer/*`. Here are some of the existing 19attributes: 20 21* :file:`length`, the total number of data samples (capacity) that can be 22 stored by the buffer. 23* :file:`enable`, activate buffer capture. 24 25IIO buffer setup 26================ 27 28The meta information associated with a channel reading placed in a buffer is 29called a scan element. The important bits configuring scan elements are 30exposed to userspace applications via the 31:file:`/sys/bus/iio/iio:device{X}/scan_elements/` directory. This directory contains 32attributes of the following form: 33 34* :file:`enable`, used for enabling a channel. If and only if its attribute 35 is non *zero*, then a triggered capture will contain data samples for this 36 channel. 37* :file:`index`, the scan_index of the channel. 38* :file:`type`, description of the scan element data storage within the buffer 39 and hence the form in which it is read from user space. 40 Format is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift] . 41 42 * *be* or *le*, specifies big or little endian. 43 * *s* or *u*, specifies if signed (2's complement) or unsigned. 44 * *bits*, is the number of valid data bits. 45 * *storagebits*, is the number of bits (after padding) that it occupies in the 46 buffer. 47 * *repeat*, specifies the number of bits/storagebits repetitions. When the 48 repeat element is 0 or 1, then the repeat value is omitted. 49 * *shift*, if specified, is the shift that needs to be applied prior to 50 masking out unused bits. 51 52For example, a driver for a 3-axis accelerometer with 12 bit resolution where 53data is stored in two 8-bits registers as follows:: 54 55 7 6 5 4 3 2 1 0 56 +---+---+---+---+---+---+---+---+ 57 |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06) 58 +---+---+---+---+---+---+---+---+ 59 60 7 6 5 4 3 2 1 0 61 +---+---+---+---+---+---+---+---+ 62 |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07) 63 +---+---+---+---+---+---+---+---+ 64 65will have the following scan element type for each axis:: 66 67 $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type 68 le:s12/16>>4 69 70A user space application will interpret data samples read from the buffer as 71two byte little endian signed data, that needs a 4 bits right shift before 72masking out the 12 valid bits of data. 73 74For implementing buffer support a driver should initialize the following 75fields in iio_chan_spec definition:: 76 77 struct iio_chan_spec { 78 /* other members */ 79 int scan_index 80 struct { 81 char sign; 82 u8 realbits; 83 u8 storagebits; 84 u8 shift; 85 u8 repeat; 86 enum iio_endian endianness; 87 } scan_type; 88 }; 89 90The driver implementing the accelerometer described above will have the 91following channel definition:: 92 93 struct iio_chan_spec accel_channels[] = { 94 { 95 .type = IIO_ACCEL, 96 .modified = 1, 97 .channel2 = IIO_MOD_X, 98 /* other stuff here */ 99 .scan_index = 0, 100 .scan_type = { 101 .sign = 's', 102 .realbits = 12, 103 .storagebits = 16, 104 .shift = 4, 105 .endianness = IIO_LE, 106 }, 107 } 108 /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1) 109 * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis 110 */ 111 } 112 113Here **scan_index** defines the order in which the enabled channels are placed 114inside the buffer. Channels with a lower **scan_index** will be placed before 115channels with a higher index. Each channel needs to have a unique 116**scan_index**. 117 118Setting **scan_index** to -1 can be used to indicate that the specific channel 119does not support buffered capture. In this case no entries will be created for 120the channel in the scan_elements directory. 121 122More details 123============ 124.. kernel-doc:: include/linux/iio/buffer.h 125.. kernel-doc:: drivers/iio/industrialio-buffer.c 126 :export: 127