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W1 linux kernel module

  • Synposis

    Rather than talk to the 1-wire bus masters directly (e.g. over a serial port or USB port), we could use the linux kernel's "w1" abstration layer.

    •  Advantages

      • Less need for external libraries (like libusb).

      • faster support for embedded GPIO masters (like the Freescale MX-27)

      • More efficient bit-banging support.

    • Disadvantages

      • Linux only (v2.6.29+)

      • Linux only

      • Rather poor performance
      • No independent control over multiple bus-masters

    • Status

      • Evgeniy Polyakov, the kernel developer,  has been kindly assisting with w1 support implementation. Design limitations have improved over time.

  • Background

    w1 is a linux kernel system that takes several types of 1-wire bus masters, and supports a subset of the 1-wire protocol on them. It seems to have been initially designed to extend the hardware monitoring function (temperature monitoring) but has evolved to support a more general 1-wire functionality. Natively, temperature readings and some memory reading is supported. Polling of 1-wire devices and values is supported.

    Without OWFS 1-wire support, there is a conflict in the interaction of w1 and OWFS. The w1 system grabs the bus-masters (specifically the USB-based DS9490) and doesn't allow OWFS access. The current work-arounds are to either

    • disable the ds9490.ko module. Probably by modifying udev rules (or hotplug in older kernels).
    • Run as root so OWFS can unload the kernel mudule.

  • Design

    • w1 grabs a list of bus masters and slaves and sends assynchronous messages as they appear in the system.
    • All bus masters are grouped together. The order is not predictable and hardware details are not exposed.
    • Slaves are grouped together and not assigned to particular bus masters.
    • Communication from user-space (OWFS) to the kernel module is handled using netlink socket. In particular the connector subtype that gives a single channel for all moduile communication.
    • OWFS uses a single thread to listen to the netlink socket, and then uses pipes internally to send the kernel messages to the relevant places in the program.

  • Status

    • w1 (now called wire1) is functional for UISB but slow.

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