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