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1=====================2HID Sensors Framework3=====================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 provides6a 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) sensor10hardware vendors to provide a consistent Plug And Play interface at the USB boundary,11thereby enabling some operating systems to incorporate common device drivers that12could be reused between vendors, alleviating any need for the vendors to provide13the drivers themselves."14 15This specification describes many usage IDs, which describe the type of sensor16and also the individual data fields. Each sensor can have variable number of17data fields. The length and order is specified in the report descriptor. For18example 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 0020.045f26 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 intensity37(0x045f) with a logical minimum value of -32767 and logical maximum of 32767. The38order of fields and length of each field is important as the input event raw39data will use this format.40 41 42Implementation43==============44 45This specification defines many different types of sensors with different sets of46data 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, whereas48an ambient light sensor can send illumination data.49So the implementation has two parts:50 51- Core HID driver52- Individual sensor processing part (sensor drivers)53 54Core driver55-----------56The core driver (hid-sensor-hub) registers as a HID driver. It parses57report descriptors and identifies all the sensors present. It adds an MFD device58with name HID-SENSOR-xxxx (where xxxx is usage id from the specification).59 60For example:61 62HID-SENSOR-200073 is registered for an Accelerometer 3D driver.63 64So if any driver with this name is inserted, then the probe routine for that65function will be called. So an accelerometer processing driver can register66with this name and will be probed if there is an accelerometer-3D detected.67 68The core driver provides a set of APIs which can be used by the processing69drivers to register and get events for that usage id. Also it provides parsing70functions, which get and set each input/feature/output report.71 72Individual sensor processing part (sensor drivers)73--------------------------------------------------74 75The processing driver will use an interface provided by the core driver to parse76the report and get the indexes of the fields and also can get events. This driver77can use IIO interface to use the standard ABI defined for a type of sensor.78 79 80Core driver Interface81=====================82 83Callback structure::84 85 Each processing driver can use this structure to set some callbacks.86 int (*suspend)(..): Callback when HID suspend is received87 int (*resume)(..): Callback when HID resume is received88 int (*capture_sample)(..): Capture a sample for one of its data fields89 int (*send_event)(..): One complete event is received which can have90 multiple data fields.91 92Registration functions::93 94 int sensor_hub_register_callback(struct hid_sensor_hub_device *hsdev,95 u32 usage_id,96 struct hid_sensor_hub_callbacks *usage_callback):97 98Registers callbacks for a usage id. The callback functions are not allowed99to sleep::100 101 102 int sensor_hub_remove_callback(struct hid_sensor_hub_device *hsdev,103 u32 usage_id):104 105Removes callbacks for a usage id.106 107 108Parsing function::109 110 int sensor_hub_input_get_attribute_info(struct hid_sensor_hub_device *hsdev,111 u8 type,112 u32 usage_id, u32 attr_usage_id,113 struct hid_sensor_hub_attribute_info *info);114 115A processing driver can look for some field of interest and check if it exists116in a report descriptor. If it exists it will store necessary information117so that fields can be set or get individually.118These indexes avoid searching every time and getting field index to get or set.119 120 121Set Feature report::122 123 int sensor_hub_set_feature(struct hid_sensor_hub_device *hsdev, u32 report_id,124 u32 field_index, s32 value);125 126This interface is used to set a value for a field in feature report. For example127if there is a field report_interval, which is parsed by a call to128sensor_hub_input_get_attribute_info before, then it can directly set that129individual field::130 131 132 int sensor_hub_get_feature(struct hid_sensor_hub_device *hsdev, u32 report_id,133 u32 field_index, s32 *value);134 135This interface is used to get a value for a field in input report. For example136if there is a field report_interval, which is parsed by a call to137sensor_hub_input_get_attribute_info before, then it can directly get that138individual field value::139 140 141 int sensor_hub_input_attr_get_raw_value(struct hid_sensor_hub_device *hsdev,142 u32 usage_id,143 u32 attr_usage_id, u32 report_id);144 145This is used to get a particular field value through input reports. For example146accelerometer wants to poll X axis value, then it can call this function with147the usage id of X axis. HID sensors can provide events, so this is not necessary148to poll for any field. If there is some new sample, the core driver will call149registered callback function to process the sample.150 151 152----------153 154HID Custom and generic Sensors155------------------------------156 157 158HID Sensor specification defines two special sensor usage types. Since they159don't represent a standard sensor, it is not possible to define using Linux IIO160type interfaces.161The purpose of these sensors is to extend the functionality or provide a162way to obfuscate the data being communicated by a sensor. Without knowing the163mapping between the data and its encapsulated form, it is difficult for164an application/driver to determine what data is being communicated by the sensor.165This allows some differentiating use cases, where vendor can provide applications.166Some common use cases are debug other sensors or to provide some events like167keyboard attached/detached or lid open/close.168 169To allow application to utilize these sensors, here they are exported using sysfs170attribute groups, attributes and misc device interface.171 172An example of this representation on sysfs::173 174 /sys/devices/pci0000:00/INT33C2:00/i2c-0/i2c-INT33D1:00/0018:8086:09FA.0001/HID-SENSOR-2000e1.6.auto$ tree -R175 .176 │ ├── enable_sensor177 │ │ ├── feature-0-200316178 │ │ │ ├── feature-0-200316-maximum179 │ │ │ ├── feature-0-200316-minimum180 │ │ │ ├── feature-0-200316-name181 │ │ │ ├── feature-0-200316-size182 │ │ │ ├── feature-0-200316-unit-expo183 │ │ │ ├── feature-0-200316-units184 │ │ │ ├── feature-0-200316-value185 │ │ ├── feature-1-200201186 │ │ │ ├── feature-1-200201-maximum187 │ │ │ ├── feature-1-200201-minimum188 │ │ │ ├── feature-1-200201-name189 │ │ │ ├── feature-1-200201-size190 │ │ │ ├── feature-1-200201-unit-expo191 │ │ │ ├── feature-1-200201-units192 │ │ │ ├── feature-1-200201-value193 │ │ ├── input-0-200201194 │ │ │ ├── input-0-200201-maximum195 │ │ │ ├── input-0-200201-minimum196 │ │ │ ├── input-0-200201-name197 │ │ │ ├── input-0-200201-size198 │ │ │ ├── input-0-200201-unit-expo199 │ │ │ ├── input-0-200201-units200 │ │ │ ├── input-0-200201-value201 │ │ ├── input-1-200202202 │ │ │ ├── input-1-200202-maximum203 │ │ │ ├── input-1-200202-minimum204 │ │ │ ├── input-1-200202-name205 │ │ │ ├── input-1-200202-size206 │ │ │ ├── input-1-200202-unit-expo207 │ │ │ ├── input-1-200202-units208 │ │ │ ├── input-1-200202-value209 210Here there is a custom sensor with four fields: two feature and two inputs.211Each field is represented by a set of attributes. All fields except the "value"212are read only. The value field is a read-write field.213 214Example::215 216 /sys/bus/platform/devices/HID-SENSOR-2000e1.6.auto/feature-0-200316$ grep -r . *217 feature-0-200316-maximum:6218 feature-0-200316-minimum:0219 feature-0-200316-name:property-reporting-state220 feature-0-200316-size:1221 feature-0-200316-unit-expo:0222 feature-0-200316-units:25223 feature-0-200316-value:1224 225How to enable such sensor?226^^^^^^^^^^^^^^^^^^^^^^^^^^227 228By default sensor can be power gated. To enable sysfs attribute "enable" can be229used::230 231 $ echo 1 > enable_sensor232 233Once enabled and powered on, sensor can report value using HID reports.234These reports are pushed using misc device interface in a FIFO order::235 236 /dev$ tree | grep HID-SENSOR-2000e1.6.auto237 │ │ │ ├── 10:53 -> ../HID-SENSOR-2000e1.6.auto238 │ ├── HID-SENSOR-2000e1.6.auto239 240Each report can be of variable length preceded by a header. This header241consists of a 32-bit usage id, 64-bit time stamp and 32-bit length field of raw242data.243