1======= 2Buffers 3======= 4 5* struct :c:type:`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 file 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:`type`, description of the scan element data storage within the buffer 38 and hence the form in which it is read from user space. 39 Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] . 40 * *be* or *le*, specifies big or little endian. 41 * *s* or *u*, specifies if signed (2's complement) or unsigned. 42 * *bits*, is the number of valid data bits. 43 * *storagebits*, is the number of bits (after padding) that it occupies in the 44 buffer. 45 * *shift*, if specified, is the shift that needs to be applied prior to 46 masking out unused bits. 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 50For example, a driver for a 3-axis accelerometer with 12 bit resolution where 51data is stored in two 8-bits registers as follows:: 52 53 7 6 5 4 3 2 1 0 54 +---+---+---+---+---+---+---+---+ 55 |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06) 56 +---+---+---+---+---+---+---+---+ 57 58 7 6 5 4 3 2 1 0 59 +---+---+---+---+---+---+---+---+ 60 |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07) 61 +---+---+---+---+---+---+---+---+ 62 63will have the following scan element type for each axis:: 64 65 $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type 66 le:s12/16>>4 67 68A user space application will interpret data samples read from the buffer as 69two byte little endian signed data, that needs a 4 bits right shift before 70masking out the 12 valid bits of data. 71 72For implementing buffer support a driver should initialize the following 73fields in iio_chan_spec definition:: 74 75 struct iio_chan_spec { 76 /* other members */ 77 int scan_index 78 struct { 79 char sign; 80 u8 realbits; 81 u8 storagebits; 82 u8 shift; 83 u8 repeat; 84 enum iio_endian endianness; 85 } scan_type; 86 }; 87 88The driver implementing the accelerometer described above will have the 89following channel definition:: 90 91 struct iio_chan_spec accel_channels[] = { 92 { 93 .type = IIO_ACCEL, 94 .modified = 1, 95 .channel2 = IIO_MOD_X, 96 /* other stuff here */ 97 .scan_index = 0, 98 .scan_type = { 99 .sign = 's', 100 .realbits = 12, 101 .storagebits = 16, 102 .shift = 4, 103 .endianness = IIO_LE, 104 }, 105 } 106 /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1) 107 * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis 108 */ 109 } 110 111Here **scan_index** defines the order in which the enabled channels are placed 112inside the buffer. Channels with a lower **scan_index** will be placed before 113channels with a higher index. Each channel needs to have a unique 114**scan_index**. 115 116Setting **scan_index** to -1 can be used to indicate that the specific channel 117does not support buffered capture. In this case no entries will be created for 118the channel in the scan_elements directory. 119 120More details 121============ 122.. kernel-doc:: include/linux/iio/buffer.h 123.. kernel-doc:: drivers/iio/industrialio-buffer.c 124 :export: 125 126