Users of the 4.2+ kernels should consider one of two patches for the I2C1 bus.  The pin muxing was not setup properly when the board was initially ported.  Users trying to use the I2C1 bus at 2.6MHz should use one of the following patches:

Torpedo:  https://patchwork.kernel.org/patch/10184763/

SOM-LV: https://patchwork.kernel.org/patch/10187657/

These patches configure the pinmux for I2C1 to disable the internal pull-up resistor and utilize the on-board resistor which should correctly allow the I2C to communicate at 2.6MHz.

Both of these patches are pending review, and should hopefully be backported to 4.4.y, 4.9.y and 4.14.y kernels in the future. We'll have a future update when this has been integrated into the kernel.

adam