On Thu, 17 Sep 2015, Tomeu Vizoso wrote:

Some buses (eg. AMBA) need access to some HW resources (it may need a
clock to be enabled so a device ID can be read) before a device can be
matched to a driver.

The pre_probe callback allows the device-driver core to request the bus
to perform this initialization and can defer the probe if any of the
resources needed are missing.

This gives us more flexibility when setting the order in which devices
are probed because the resources needed to get the matching information
don't need to be available by the time that the bus devices are
registered.

Can't the subsystem do this itself in its matching routine?

Alan Stern