The 1-wire (w1) subsystem
  ------------------------------------------------------------------
  The 1-wire bus is a simple master-slave bus that communicates via a single
  signal wire (plus ground, so two wires).
  
  Devices communicate on the bus by pulling the signal to ground via an open
  drain output and by sampling the logic level of the signal line.
  
  The w1 subsystem provides the framework for managing w1 masters and
  communication with slaves.
  
  All w1 slave devices must be connected to a w1 bus master device.
  
  Example w1 master devices:
      DS9490 usb device
      W1-over-GPIO
      DS2482 (i2c to w1 bridge)
      Emulated devices, such as a RS232 converter, parallel port adapter, etc
  
  
  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 family
  and 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 perform
  almost any kind of operations. Each logical operation is a transaction
  in 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 byte
  and two byte address. At this step bus is reset and appropriate device
  is 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 searching
  and slave device will be even removed, but in this case 0xff will
  be 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) and
  registers with the w1 subsystem.
  
  Current family drivers:
  w1_therm - (ds18?20 thermal sensor family driver)
      provides temperature reading function which is bound to ->rbin() method
      of 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 and
  sample 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
  ------------------------------------------------------------------

  <xx-xxxxxxxxxxxx>  - A directory for a found device. The format is family-serial

  bus                - (standard) symlink to the w1 bus
  driver             - (standard) symlink to the w1 driver

  w1_master_add      - (rw) manually register a slave device
  w1_master_attempts - (ro) the number of times a search was attempted

  w1_master_max_slave_count

                     - (rw) maximum number of slaves to search for at a time
  w1_master_name     - (ro) the name of the device (w1_bus_masterX)
  w1_master_pullup   - (rw) 5V strong pullup 0 enabled, 1 disabled
  w1_master_remove   - (rw) manually remove a slave device
  w1_master_search   - (rw) the number of searches left to do,
  		     -1=continual (default)

  w1_master_slave_count

                     - (ro) the number of slaves found
  w1_master_slaves   - (ro) the names of the slaves, one per line
  w1_master_timeout  - (ro) the delay in seconds between searches
  w1_master_timeout_us
                     - (ro) the delay in microseconds beetwen searches

  
  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 number
  for an initially small number of bus searches.  Alternatively it could be
  set to zero, then manually add the slave device serial numbers by
  w1_master_add device file.  The w1_master_add and w1_master_remove files
  generally only make sense when searching is disabled, as a search will
  redetect manually removed devices that are present and timeout manually
  added devices that aren't on the bus.

  Bus searches occur at an interval, specified as a summ of timeout and
  timeout_us module parameters (either of which may be 0) for as long as
  w1_master_search remains greater than 0 or is -1.  Each search attempt
  decrements w1_master_search by 1 (down to 0) and increments
  w1_master_attempts by 1.

  
  w1 slave sysfs interface
  ------------------------------------------------------------------
  bus                - (standard) symlink to the w1 bus
  driver             - (standard) symlink to the w1 driver
  name               - the device name, usually the same as the directory name
  w1_slave           - (optional) a binary file whose meaning depends on the
                       family driver

  rw		   - (optional) created for slave devices which do not have
  		     appropriate family driver. Allows to read/write binary data.