Documentation/driver-api/iio/buffers.rst

   1 =======
   2 Buffers
   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
  11 The Industrial I/O core offers a way for continuous data capture based on a
  12 trigger 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
  15 IIO buffer sysfs interface
  16 ==========================
  17 An IIO buffer has an associated attributes directory under
  18 :file:`/sys/bus/iio/iio:device{X}/buffer/*`. Here are some of the existing
  19 attributes:
  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
  25 IIO buffer setup
  26 ================
  27
  28 The meta information associated with a channel reading placed in a buffer is
  29 called a scan element. The important bits configuring scan elements are
  30 exposed to userspace applications via the
  31 :file:`/sys/bus/iio/iio:device{X}/scan_elements/*` directory. This file contains
  32 attributes 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
  50 For example, a driver for a 3-axis accelerometer with 12 bit resolution where
  51 data 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
  63 will 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
  68 A user space application will interpret data samples read from the buffer as
  69 two byte little endian signed data, that needs a 4 bits right shift before
  70 masking out the 12 valid bits of data.
  71
  72 For implementing buffer support a driver should initialize the following
  73 fields 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
  88 The driver implementing the accelerometer described above will have the
  89 following channel definition::
  90
  91    struct 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
 111 Here **scan_index** defines the order in which the enabled channels are placed
 112 inside the buffer. Channels with a lower **scan_index** will be placed before
 113 channels with a higher index. Each channel needs to have a unique
 114 **scan_index**.
 115
 116 Setting **scan_index** to -1 can be used to indicate that the specific channel
 117 does not support buffered capture. In this case no entries will be created for
 118 the channel in the scan_elements directory.
 119
 120 More details
 121 ============
 122 .. kernel-doc:: include/linux/iio/buffer.h
 123 .. kernel-doc:: drivers/iio/industrialio-buffer.c
 124    :export:
 125