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
142
143----------
144
145HID Custom and generic Sensors
146
147HID Sensor specification defines two special sensor usage types. Since they
148don't represent a standard sensor, it is not possible to define using Linux IIO
149type interfaces.
150The purpose of these sensors is to extend the functionality or provide a
151way to obfuscate the data being communicated by a sensor. Without knowing the
152mapping between the data and its encapsulated form, it is difficult for
153an application/driver to determine what data is being communicated by the sensor.
154This allows some differentiating use cases, where vendor can provide applications.
155Some common use cases are debug other sensors or to provide some events like
156keyboard attached/detached or lid open/close.
157
158To allow application to utilize these sensors, here they are exported uses sysfs
159attribute groups, attributes and misc device interface.
160
161An example of this representation on sysfs:
162/sys/devices/pci0000:00/INT33C2:00/i2c-0/i2c-INT33D1:00/0018:8086:09FA.0001/HID-SENSOR-2000e1.6.auto$ tree -R
163.
164????????? enable_sensor
165????????? feature-0-200316
166??????? ????????? feature-0-200316-maximum
167??????? ????????? feature-0-200316-minimum
168??????? ????????? feature-0-200316-name
169??????? ????????? feature-0-200316-size
170??????? ????????? feature-0-200316-unit-expo
171??????? ????????? feature-0-200316-units
172??????? ????????? feature-0-200316-value
173????????? feature-1-200201
174??????? ????????? feature-1-200201-maximum
175??????? ????????? feature-1-200201-minimum
176??????? ????????? feature-1-200201-name
177??????? ????????? feature-1-200201-size
178??????? ????????? feature-1-200201-unit-expo
179??????? ????????? feature-1-200201-units
180??????? ????????? feature-1-200201-value
181????????? input-0-200201
182??????? ????????? input-0-200201-maximum
183??????? ????????? input-0-200201-minimum
184??????? ????????? input-0-200201-name
185??????? ????????? input-0-200201-size
186??????? ????????? input-0-200201-unit-expo
187??????? ????????? input-0-200201-units
188??????? ????????? input-0-200201-value
189????????? input-1-200202
190??????? ????????? input-1-200202-maximum
191??????? ????????? input-1-200202-minimum
192??????? ????????? input-1-200202-name
193??????? ????????? input-1-200202-size
194??????? ????????? input-1-200202-unit-expo
195??????? ????????? input-1-200202-units
196??????? ????????? input-1-200202-value
197
198Here there is a custom sensors with four fields, two feature and two inputs.
199Each field is represented by a set of attributes. All fields except the "value"
200are read only. The value field is a RW field.
201Example
202/sys/bus/platform/devices/HID-SENSOR-2000e1.6.auto/feature-0-200316$ grep -r . *
203feature-0-200316-maximum:6
204feature-0-200316-minimum:0
205feature-0-200316-name:property-reporting-state
206feature-0-200316-size:1
207feature-0-200316-unit-expo:0
208feature-0-200316-units:25
209feature-0-200316-value:1
210
211How to enable such sensor?
212By default sensor can be power gated. To enable sysfs attribute "enable" can be
213used.
214$ echo 1 > enable_sensor
215
216Once enabled and powered on, sensor can report value using HID reports.
217These reports are pushed using misc device interface in a FIFO order.
218/dev$ tree | grep HID-SENSOR-2000e1.6.auto
219??????? ????????? 10:53 -> ../HID-SENSOR-2000e1.6.auto
220????????? HID-SENSOR-2000e1.6.auto
221
222Each reports can be of variable length preceded by a header. This header
223consist of a 32 bit usage id, 64 bit time stamp and 32 bit length field of raw
224data.
225