On Fri, May 23, 2014 at 11:13:44AM +0200, Hans de Goede wrote:

Thinking more about this, I would like to make one change to my
proposal, the mmc-core should only do power up of child-nodes if
they have a compatible of: "simple-sdio-powerup". This way
when we add something more complex, we can keep the simple powerup
code in the mmc core, keeping what we've already using this working
and the mmc core won't respond to the child nodes for more complex
devices, so it won't conflict with more complex power-up handling
handled by some other driver.

Would it not be better to have this be something in the driver struct
rather than in the device tree?  Putting a compatible in there would be
encoding details of the Linux implementation in the DT which doesn't
seem right especially since these are details we're thinking of changing
later on.  Something like have the driver set flags saying if it wants
to do complicated things.

FWIW if we ever get truely complex cases I think modeling the
power-up hardware as a pmic platform device is not a bad idea,
we would then need to have a generic mmc-host pmic property, which
would be used both to do the initial powerup before scanning, as
well as for the sdio device driver to get a handle to the pmic,
for run time power-management (if desired).

I don't know if this will ever apply to SDIO but with other buses the
complicated bits come when the driver wants to take over some of the
power management do things like turn some of the supplies or clocks on
and off independently at runtime for low power modes.