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probe() determines whether the device corresponding
to dip actually exists and is a valid device
for this driver. probe() is called after identify(9E)
and before attach(9E) for a given dip. For example, the probe() routine can
map the device registers using ddi_map_regs(9F)
then attempt to access the hardware using ddi_peek(9F) or ddi_poke(9F) and determine if the device
exists. Then the device registers should be unmapped using ddi_unmap_regs(9F).
To probe a device that was left powered off after the last detach(), it might be necessary to power it up. If so, the driver
must power up the device by accessing device registers directly. pm_raise_power(9F)
will be not be available until attach(9E).
The framework ensures that the ancestors of the node being probed and all
relevant platform-specific power management hardware is at full power at
the time that probe() is called.
probe() should only probe the device. It should
not change any software state and should not create any software state.
Device initialization should be done in attach(9E).
For a self-identifying device, this entry point is not necessary.
However, if a device exists in both self-identifying and non-self-identifying
forms, a probe() routine can be provided to simplify
the driver. ddi_dev_is_sid(9F)
can then be used to determine whether probe() needs
to do any work. See ddi_dev_is_sid(9F)
for an example.
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