The hardware does not expose a classic SMBus interface.
Instead it has a lower level interface that can express a far richer
i2c protocol than what smbus offers. However, the interface does not
provide a way to explicitly generate the i2c stop and start conditions.
It's only possible to request that the stop condition is generated
after transferring the next byte in either direction. So, at least
one data byte must always be transferred.
Thus, some i2c sequences are impossible to generate, e.g., an equivalent
of smbus quick command (<start>-<slave addr>-<r/w bit>-<stop>).
At the same time isl(4) and cyapa(4) are moved to iicbus and now they use
iicbus_transfer for communication. Previously they used smbus_trans()
interface that is not defined by the SMBus protocol and was implemented
only by ig4(4). In fact, that interface was impossible to implement
for the typical SMBus controllers (like intpm(4) or ichsmb(4)) where
a type of the SMBus command must be programmed.
The plan is to remove smbus_trans() and all its uses.
As an aside, the smbus_trans() method deviates from the standard,
but perhaps backwards, FreeBSD convention of using 8-bit slave
addresses (shifted by 1 bit to the left). The method expects
7-bit addresses.
There is a user facing consequence of this change: the user now must
provide device hints for isl and cyapa that specify an iicbus to use
and a slave address on it.
It would be nice to make these drivers more plug-and-play.
Linux has a special platform driver that uses DMI information to attach
drivers to specific buses and addresses. We could have something like
that too.
Right now smbus(4) driver tries to discover all slaves on the bus.
That is very dangerous. Fortunately, the probing code uses smbus_trans()
to do its job, so it is really enabled for ig4 only.
The plan is to remove that auto-probing code.