On Fri, Jul 11, 2014 at 8:29 AM, Tuomas Tynkkynen [off-list ref] wrote:

On the hardware level, the two I2C controllers sharing the same pins
have knowledge of each other and won't start transmitting if the bus
is busy (something different from the usual I2C arbitration, that is).
I guess on the kernel side there could be a problem if the voltage register
is marked cached in the PMIC driver's regmap.

Yeah, in our tree we have a hack to disable regcache for SD0_VOLTAGE.
Other than the value reported to userspace being wrong, leaving it as
cacheable shouldn't be an issue if no other drivers try to read/write
that register.