Reading the file will give you a list of masters which can be selected for a demultiplexed bus. The format is "<index>:<name>". Example from a Renesas Lager board:
What: /sys/devices/platform/<i2c-demux-name>/current_masterDate: January 2016KernelVersion: 4.6Contact: Wolfram Sang <wsa@the-dreams.de>Description: This file selects/shows the active I2C master for a demultiplexed bus. It uses the <index> value from the file 'available_masters'.
