• Simon Glass's avatar
    i2c: Add a mux for GPIO-based I2C bus arbitration · b725dc45
    Simon Glass authored
    While I2C supports multi-master buses this is difficult to get right.
    The implementation on the master side in software is quite complex.
    Clock-stretching and the arbitrary time that an I2C transaction can take
    make it difficult to share the bus fairly in the face of high traffic.
    When one or more masters can be reset independently part-way through a
    transaction it is hard to know the state of the bus.
    This driver provides a scheme based on two 'claim' GPIOs, one driven by the
    AP (Application Processor, meaning the main CPU) and one driven by the EC
    (Embedded Controller, a small CPU aimed at handling system tasks). With
    these they can communicate and reliably share the bus. This scheme has
    minimal overhead and involves very little code. It is used on snow to
    permit the EC and the AP to share access to the main system PMIC and
    battery. The scheme can survive reboots by either side without difficulty.
    This scheme has been tested in the field with millions of devices.
    Since U-Boot runs on the AP, the terminology used is 'our' claim GPIO,
    meaning the AP's, and 'their' claim GPIO, meaning the EC's. This terminology
    is used by the device tree bindings in Linux also.
    Signed-off-by: default avatarSimon Glass <sjg@chromium.org>
README.i2c 2.58 KB