linux/Documentation/hid/hid-sensor.txt
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   1
   2HID Sensors Framework
   3======================
   4HID sensor framework provides necessary interfaces to implement sensor drivers,
   5which are connected to a sensor hub. The sensor hub is a HID device and it provides
   6a report descriptor conforming to HID 1.12 sensor usage tables.
   7
   8Description from the HID 1.12 "HID Sensor Usages" specification:
   9"Standardization of HID usages for sensors would allow (but not require) sensor
  10hardware vendors to provide a consistent Plug And Play interface at the USB boundary,
  11thereby enabling some operating systems to incorporate common device drivers that
  12could be reused between vendors, alleviating any need for the vendors to provide
  13the drivers themselves."
  14
  15This specification describes many usage IDs, which describe the type of sensor
  16and also the individual data fields. Each sensor can have variable number of
  17data fields. The length and order is specified in the report descriptor. For
  18example a part of report descriptor can look like:
  19
  20   INPUT(1)[INPUT]
  21 ..
  22    Field(2)
  23      Physical(0020.0073)
  24      Usage(1)
  25        0020.045f
  26      Logical Minimum(-32767)
  27      Logical Maximum(32767)
  28      Report Size(8)
  29      Report Count(1)
  30      Report Offset(16)
  31      Flags(Variable Absolute)
  32..
  33..
  34
  35The report is indicating "sensor page (0x20)" contains an accelerometer-3D (0x73).
  36This accelerometer-3D has some fields. Here for example field 2 is motion intensity
  37(0x045f) with a logical minimum value of -32767 and logical maximum of 32767. The
  38order of fields and length of each field is important as the input event raw
  39data will use this format.
  40
  41
  42Implementation
  43=================
  44
  45This specification defines many different types of sensors with different sets of
  46data fields. It is difficult to have a common input event to user space applications,
  47for different sensors. For example an accelerometer can send X,Y and Z data, whereas
  48an ambient light sensor can send illumination data.
  49So the implementation has two parts:
  50- Core hid driver
  51- Individual sensor processing part (sensor drivers)
  52
  53Core driver
  54-----------
  55The core driver registers (hid-sensor-hub) registers as a HID driver. It parses
  56report descriptors and identifies all the sensors present. It adds an MFD device
  57with name HID-SENSOR-xxxx (where xxxx is usage id from the specification).
  58For example
  59HID-SENSOR-200073 is registered for an Accelerometer 3D driver.
  60So if any driver with this name is inserted, then the probe routine for that
  61function will be called. So an accelerometer processing driver can register
  62with this name and will be probed if there is an accelerometer-3D detected.
  63
  64The core driver provides a set of APIs which can be used by the processing
  65drivers to register and get events for that usage id. Also it provides parsing
  66functions, which get and set each input/feature/output report.
  67
  68Individual sensor processing part (sensor drivers)
  69-----------
  70The processing driver will use an interface provided by the core driver to parse
  71the report and get the indexes of the fields and also can get events. This driver
  72can use IIO interface to use the standard ABI defined for a type of sensor.
  73
  74
  75Core driver Interface
  76=====================
  77
  78Callback structure:
  79Each processing driver can use this structure to set some callbacks.
  80        int (*suspend)(..): Callback when HID suspend is received
  81        int (*resume)(..): Callback when HID resume is received
  82        int (*capture_sample)(..): Capture a sample for one of its data fields
  83        int (*send_event)(..): One complete event is received which can have
  84                               multiple data fields.
  85
  86Registration functions:
  87int sensor_hub_register_callback(struct hid_sensor_hub_device *hsdev,
  88                        u32 usage_id,
  89                        struct hid_sensor_hub_callbacks *usage_callback):
  90
  91Registers callbacks for an usage id. The callback functions are not allowed
  92to sleep.
  93
  94
  95int sensor_hub_remove_callback(struct hid_sensor_hub_device *hsdev,
  96                        u32 usage_id):
  97
  98Removes callbacks for an usage id.
  99
 100
 101Parsing function:
 102int sensor_hub_input_get_attribute_info(struct hid_sensor_hub_device *hsdev,
 103                        u8 type,
 104                        u32 usage_id, u32 attr_usage_id,
 105                        struct hid_sensor_hub_attribute_info *info);
 106
 107A processing driver can look for some field of interest and check if it exists
 108in a report descriptor. If it exists it will store necessary information
 109so that fields can be set or get individually.
 110These indexes avoid searching every time and getting field index to get or set.
 111
 112
 113Set Feature report
 114int sensor_hub_set_feature(struct hid_sensor_hub_device *hsdev, u32 report_id,
 115                        u32 field_index, s32 value);
 116
 117This interface is used to set a value for a field in feature report. For example
 118if there is a field report_interval, which is parsed by a call to
 119sensor_hub_input_get_attribute_info before, then it can directly set that individual
 120field.
 121
 122
 123int sensor_hub_get_feature(struct hid_sensor_hub_device *hsdev, u32 report_id,
 124                        u32 field_index, s32 *value);
 125
 126This interface is used to get a value for a field in input report. For example
 127if there is a field report_interval, which is parsed by a call to
 128sensor_hub_input_get_attribute_info before, then it can directly get that individual
 129field value.
 130
 131
 132int sensor_hub_input_attr_get_raw_value(struct hid_sensor_hub_device *hsdev,
 133                        u32 usage_id,
 134                        u32 attr_usage_id, u32 report_id);
 135
 136This is used to get a particular field value through input reports. For example
 137accelerometer wants to poll X axis value, then it can call this function with
 138the usage id of X axis. HID sensors can provide events, so this is not necessary
 139to poll for any field. If there is some new sample, the core driver will call
 140registered callback function to process the sample.
 141
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