What does the w1 subsystem do?¶

When a w1 master driver registers with the w1 subsystem, the following occurs:

• sysfs entries for that w1 master are created
• the w1 bus is periodically searched for new slave devices

When a device is found on the bus, w1 core tries to load the driver for its familyand check if it is loaded. If so, the family driver is attached to the slave.If there is no driver for the family, default one is assigned, which allows to performalmost any kind of operations. Each logical operation is a transactionin nature, which can contain several (two or one) low-level operations.Let’s see how one can read EEPROM context:1. one must write control buffer, i.e. buffer containing command byteand two byte address. At this step bus is reset and appropriate deviceis selected using either W1_SKIP_ROM or W1_MATCH_ROM command.Then provided control buffer is being written to the wire.2. reading. This will issue reading eeprom response.

It is possible that between 1. and 2. w1 master thread will reset bus for searchingand slave device will be even removed, but in this case 0xff willbe read, since no device was selected.

W1 device families¶

Slave devices are handled by a driver written for a family of w1 devices.

A family driver populates a struct w1_family_ops (see w1_family.h) andregisters with the w1 subsystem.

Current family drivers:

w1_therm

• (ds18?20 thermal sensor family driver)provides temperature reading function which is bound to ->rbin() methodof the above w1_family_ops structure.

w1_smem

• driver for simple 64bit memory cell provides ID reading method.

You can call above methods by reading appropriate sysfs files.

What does a w1 master driver need to implement?¶

The driver for w1 bus master must provide at minimum two functions.

Emulated devices must provide the ability to set the output signal level(write_bit) and sample the signal level (read_bit).

Devices that support the 1-wire natively must provide the ability to write andsample a bit (touch_bit) and reset the bus (reset_bus).

Most hardware provides higher-level functions that offload w1 handling.See struct w1_bus_master definition in w1.h for details.

w1 master sysfs interface¶

If you have a w1 bus that never changes (you don’t add or remove devices),you can set the module parameter search_count to a small positive numberfor an initially small number of bus searches. Alternatively it could beset to zero, then manually add the slave device serial numbers byw1_master_add device file. The w1_master_add and w1_master_remove filesgenerally only make sense when searching is disabled, as a search willredetect manually removed devices that are present and timeout manuallyadded devices that aren’t on the bus.

Bus searches occur at an interval, specified as a summ of timeout andtimeout_us module parameters (either of which may be 0) for as long asw1_master_search remains greater than 0 or is -1. Each search attemptdecrements w1_master_search by 1 (down to 0) and incrementsw1_master_attempts by 1.

w1 slave sysfs interface¶