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diff --git a/Documentation/hid/hid-sensor.txt b/Documentation/hid/hid-sensor.txt new file mode 100644 index 00000000..948b0989 --- /dev/null +++ b/Documentation/hid/hid-sensor.txt @@ -0,0 +1,140 @@ + +HID Sensors Framework +====================== +HID sensor framework provides necessary interfaces to implement sensor drivers, +which are connected to a sensor hub. The sensor hub is a HID device and it provides +a report descriptor conforming to HID 1.12 sensor usage tables. + +Description from the HID 1.12 "HID Sensor Usages" specification: +"Standardization of HID usages for sensors would allow (but not require) sensor +hardware vendors to provide a consistent Plug And Play interface at the USB boundary, +thereby enabling some operating systems to incorporate common device drivers that +could be reused between vendors, alleviating any need for the vendors to provide +the drivers themselves." + +This specification describes many usage IDs, which describe the type of sensor +and also the individual data fields. Each sensor can have variable number of +data fields. The length and order is specified in the report descriptor. For +example a part of report descriptor can look like: + + INPUT(1)[INPUT] + .. + Field(2) + Physical(0020.0073) + Usage(1) + 0020.045f + Logical Minimum(-32767) + Logical Maximum(32767) + Report Size(8) + Report Count(1) + Report Offset(16) + Flags(Variable Absolute) +.. +.. + +The report is indicating "sensor page (0x20)" contains an accelerometer-3D (0x73). +This accelerometer-3D has some fields. Here for example field 2 is motion intensity +(0x045f) with a logical minimum value of -32767 and logical maximum of 32767. The +order of fields and length of each field is important as the input event raw +data will use this format. + + +Implementation +================= + +This specification defines many different types of sensors with different sets of +data fields. It is difficult to have a common input event to user space applications, +for different sensors. For example an accelerometer can send X,Y and Z data, whereas +an ambient light sensor can send illumination data. +So the implementation has two parts: +- Core hid driver +- Individual sensor processing part (sensor drivers) + +Core driver +----------- +The core driver registers (hid-sensor-hub) registers as a HID driver. It parses +report descriptors and identifies all the sensors present. It adds an MFD device +with name HID-SENSOR-xxxx (where xxxx is usage id from the specification). +For example +HID-SENSOR-200073 is registered for an Accelerometer 3D driver. +So if any driver with this name is inserted, then the probe routine for that +function will be called. So an accelerometer processing driver can register +with this name and will be probed if there is an accelerometer-3D detected. + +The core driver provides a set of APIs which can be used by the processing +drivers to register and get events for that usage id. Also it provides parsing +functions, which get and set each input/feature/output report. + +Individual sensor processing part (sensor drivers) +----------- +The processing driver will use an interface provided by the core driver to parse +the report and get the indexes of the fields and also can get events. This driver +can use IIO interface to use the standard ABI defined for a type of sensor. + + +Core driver Interface +===================== + +Callback structure: +Each processing driver can use this structure to set some callbacks. + int (*suspend)(..): Callback when HID suspend is received + int (*resume)(..): Callback when HID resume is received + int (*capture_sample)(..): Capture a sample for one of its data fields + int (*send_event)(..): One complete event is received which can have + multiple data fields. + +Registration functions: +int sensor_hub_register_callback(struct hid_sensor_hub_device *hsdev, + u32 usage_id, + struct hid_sensor_hub_callbacks *usage_callback): + +Registers callbacks for an usage id. The callback functions are not allowed +to sleep. + + +int sensor_hub_remove_callback(struct hid_sensor_hub_device *hsdev, + u32 usage_id): + +Removes callbacks for an usage id. + + +Parsing function: +int sensor_hub_input_get_attribute_info(struct hid_sensor_hub_device *hsdev, + u8 type, + u32 usage_id, u32 attr_usage_id, + struct hid_sensor_hub_attribute_info *info); + +A processing driver can look for some field of interest and check if it exists +in a report descriptor. If it exists it will store necessary information +so that fields can be set or get individually. +These indexes avoid searching every time and getting field index to get or set. + + +Set Feature report +int sensor_hub_set_feature(struct hid_sensor_hub_device *hsdev, u32 report_id, + u32 field_index, s32 value); + +This interface is used to set a value for a field in feature report. For example +if there is a field report_interval, which is parsed by a call to +sensor_hub_input_get_attribute_info before, then it can directly set that individual +field. + + +int sensor_hub_get_feature(struct hid_sensor_hub_device *hsdev, u32 report_id, + u32 field_index, s32 *value); + +This interface is used to get a value for a field in input report. For example +if there is a field report_interval, which is parsed by a call to +sensor_hub_input_get_attribute_info before, then it can directly get that individual +field value. + + +int sensor_hub_input_attr_get_raw_value(struct hid_sensor_hub_device *hsdev, + u32 usage_id, + u32 attr_usage_id, u32 report_id); + +This is used to get a particular field value through input reports. For example +accelerometer wants to poll X axis value, then it can call this function with +the usage id of X axis. HID sensors can provide events, so this is not necessary +to poll for any field. If there is some new sample, the core driver will call +registered callback function to process the sample. |