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
51- Core HID driver
52- Individual sensor processing part (sensor drivers)
53
54Core driver
55-----------
56The core driver (hid-sensor-hub) registers as a HID driver. It parses
57report descriptors and identifies all the sensors present. It adds an MFD device
58with 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 that
65function will be called. So an accelerometer processing driver can register
66with 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 processing
69drivers to register and get events for that usage id. Also it provides parsing
70functions, 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 parse
76the report and get the indexes of the fields and also can get events. This driver
77can use IIO interface to use the standard ABI defined for a type of sensor.
78
79
80Core driver Interface
81=====================
82
83Callback structure::
84
85  Each processing driver can use this structure to set some callbacks.
86	int (*suspend)(..): Callback when HID suspend is received
87	int (*resume)(..): Callback when HID resume is received
88	int (*capture_sample)(..): Capture a sample for one of its data fields
89	int (*send_event)(..): One complete event is received which can have
90                               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 allowed
99to 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 exists
116in a report descriptor. If it exists it will store necessary information
117so 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 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 set that
129individual 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 example
136if there is a field report_interval, which is parsed by a call to
137sensor_hub_input_get_attribute_info before, then it can directly get that
138individual 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 example
146accelerometer wants to poll X axis value, then it can call this function with
147the usage id of X axis. HID sensors can provide events, so this is not necessary
148to poll for any field. If there is some new sample, the core driver will call
149registered callback function to process the sample.
150
151
152----------
153
154HID Custom and generic Sensors
155------------------------------
156
157
158HID Sensor specification defines two special sensor usage types. Since they
159don't represent a standard sensor, it is not possible to define using Linux IIO
160type interfaces.
161The purpose of these sensors is to extend the functionality or provide a
162way to obfuscate the data being communicated by a sensor. Without knowing the
163mapping between the data and its encapsulated form, it is difficult for
164an 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 like
167keyboard attached/detached or lid open/close.
168
169To allow application to utilize these sensors, here they are exported using sysfs
170attribute 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 -R
175  .
176  │   ├──  enable_sensor
177  │   │   ├── feature-0-200316
178  │   │   │   ├── feature-0-200316-maximum
179  │   │   │   ├── feature-0-200316-minimum
180  │   │   │   ├── feature-0-200316-name
181  │   │   │   ├── feature-0-200316-size
182  │   │   │   ├── feature-0-200316-unit-expo
183  │   │   │   ├── feature-0-200316-units
184  │   │   │   ├── feature-0-200316-value
185  │   │   ├── feature-1-200201
186  │   │   │   ├── feature-1-200201-maximum
187  │   │   │   ├── feature-1-200201-minimum
188  │   │   │   ├── feature-1-200201-name
189  │   │   │   ├── feature-1-200201-size
190  │   │   │   ├── feature-1-200201-unit-expo
191  │   │   │   ├── feature-1-200201-units
192  │   │   │   ├── feature-1-200201-value
193  │   │   ├── input-0-200201
194  │   │   │   ├── input-0-200201-maximum
195  │   │   │   ├── input-0-200201-minimum
196  │   │   │   ├── input-0-200201-name
197  │   │   │   ├── input-0-200201-size
198  │   │   │   ├── input-0-200201-unit-expo
199  │   │   │   ├── input-0-200201-units
200  │   │   │   ├── input-0-200201-value
201  │   │   ├── input-1-200202
202  │   │   │   ├── input-1-200202-maximum
203  │   │   │   ├── input-1-200202-minimum
204  │   │   │   ├── input-1-200202-name
205  │   │   │   ├── input-1-200202-size
206  │   │   │   ├── input-1-200202-unit-expo
207  │   │   │   ├── input-1-200202-units
208  │   │   │   ├── input-1-200202-value
209
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:6
218  feature-0-200316-minimum:0
219  feature-0-200316-name:property-reporting-state
220  feature-0-200316-size:1
221  feature-0-200316-unit-expo:0
222  feature-0-200316-units:25
223  feature-0-200316-value:1
224
225How to enable such sensor?
226^^^^^^^^^^^^^^^^^^^^^^^^^^
227
228By default sensor can be power gated. To enable sysfs attribute "enable" can be
229used::
230
231	$ echo 1 > enable_sensor
232
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.auto
237	│   │   │   ├── 10:53 -> ../HID-SENSOR-2000e1.6.auto
238	│   ├──  HID-SENSOR-2000e1.6.auto
239
240Each report can be of variable length preceded by a header. This header
241consists of a 32-bit usage id, 64-bit time stamp and 32-bit length field of raw
242data.
243