1The 1-wire (w1) subsystem
2------------------------------------------------------------------
3The 1-wire bus is a simple master-slave bus that communicates via a single
4signal wire (plus ground, so two wires).
5
6Devices communicate on the bus by pulling the signal to ground via an open
7drain output and by sampling the logic level of the signal line.
8
9The w1 subsystem provides the framework for managing w1 masters and
10communication with slaves.
11
12All w1 slave devices must be connected to a w1 bus master device.
13
14Example w1 master devices:
15    DS9490 usb device
16    W1-over-GPIO
17    DS2482 (i2c to w1 bridge)
18    Emulated devices, such as a RS232 converter, parallel port adapter, etc
19
20
21What does the w1 subsystem do?
22------------------------------------------------------------------
23When a w1 master driver registers with the w1 subsystem, the following occurs:
24
25 - sysfs entries for that w1 master are created
26 - the w1 bus is periodically searched for new slave devices
27
28When a device is found on the bus, w1 core tries to load the driver for its family
29and check if it is loaded. If so, the family driver is attached to the slave.
30If there is no driver for the family, default one is assigned, which allows to perform
31almost any kind of operations. Each logical operation is a transaction
32in nature, which can contain several (two or one) low-level operations.
33Let's see how one can read EEPROM context:
341. one must write control buffer, i.e. buffer containing command byte
35and two byte address. At this step bus is reset and appropriate device
36is selected using either W1_SKIP_ROM or W1_MATCH_ROM command.
37Then provided control buffer is being written to the wire.
382. reading. This will issue reading eeprom response.
39
40It is possible that between 1. and 2. w1 master thread will reset bus for searching
41and slave device will be even removed, but in this case 0xff will
42be read, since no device was selected.
43
44
45W1 device families
46------------------------------------------------------------------
47Slave devices are handled by a driver written for a family of w1 devices.
48
49A family driver populates a struct w1_family_ops (see w1_family.h) and
50registers with the w1 subsystem.
51
52Current family drivers:
53w1_therm - (ds18?20 thermal sensor family driver)
54    provides temperature reading function which is bound to ->rbin() method
55    of the above w1_family_ops structure.
56
57w1_smem - driver for simple 64bit memory cell provides ID reading method.
58
59You can call above methods by reading appropriate sysfs files.
60
61
62What does a w1 master driver need to implement?
63------------------------------------------------------------------
64
65The driver for w1 bus master must provide at minimum two functions.
66
67Emulated devices must provide the ability to set the output signal level
68(write_bit) and sample the signal level (read_bit).
69
70Devices that support the 1-wire natively must provide the ability to write and
71sample a bit (touch_bit) and reset the bus (reset_bus).
72
73Most hardware provides higher-level functions that offload w1 handling.
74See struct w1_bus_master definition in w1.h for details.
75
76
77w1 master sysfs interface
78------------------------------------------------------------------
79<xx-xxxxxxxxxxxxx> - A directory for a found device. The format is family-serial
80bus                - (standard) symlink to the w1 bus
81driver             - (standard) symlink to the w1 driver
82w1_master_add      - (rw) manually register a slave device
83w1_master_attempts - (ro) the number of times a search was attempted
84w1_master_max_slave_count
85                   - (rw) maximum number of slaves to search for at a time
86w1_master_name     - (ro) the name of the device (w1_bus_masterX)
87w1_master_pullup   - (rw) 5V strong pullup 0 enabled, 1 disabled
88w1_master_remove   - (rw) manually remove a slave device
89w1_master_search   - (rw) the number of searches left to do,
90		     -1=continual (default)
91w1_master_slave_count
92                   - (ro) the number of slaves found
93w1_master_slaves   - (ro) the names of the slaves, one per line
94w1_master_timeout  - (ro) the delay in seconds between searches
95w1_master_timeout_us
96                   - (ro) the delay in microseconds beetwen searches
97
98If you have a w1 bus that never changes (you don't add or remove devices),
99you can set the module parameter search_count to a small positive number
100for an initially small number of bus searches.  Alternatively it could be
101set to zero, then manually add the slave device serial numbers by
102w1_master_add device file.  The w1_master_add and w1_master_remove files
103generally only make sense when searching is disabled, as a search will
104redetect manually removed devices that are present and timeout manually
105added devices that aren't on the bus.
106
107Bus searches occur at an interval, specified as a summ of timeout and
108timeout_us module parameters (either of which may be 0) for as long as
109w1_master_search remains greater than 0 or is -1.  Each search attempt
110decrements w1_master_search by 1 (down to 0) and increments
111w1_master_attempts by 1.
112
113w1 slave sysfs interface
114------------------------------------------------------------------
115bus                - (standard) symlink to the w1 bus
116driver             - (standard) symlink to the w1 driver
117name               - the device name, usually the same as the directory name
118w1_slave           - (optional) a binary file whose meaning depends on the
119                     family driver
120rw		   - (optional) created for slave devices which do not have
121		     appropriate family driver. Allows to read/write binary data.
122