[PATCH 1/3] mmc: add support for power-on sequencing through DT

On Tue, Jan 28, 2014 at 11:48:10AM +0100, Arnd Bergmann wrote:

I think there is another option, which does have its own pros and cons:
We could move all the power handling back into the sdio function driver
if we allow a secondary detection path using DT rather than the probing
of the SDIO bus. Essentially you'd have to list the class/vendor/device
ID for each function that cannot be autodetected in DT, and have the
SDIO core pretend that it found the device just by looking at the
device nodes, and register the struct sdio_func so it can be bound to
the driver. The driver then does all the power handling in a device
specific way. At some point the hardware gets registered at the 
mmc host, and the sdio core connects the bus state to the already present
sdio_func, possibly notifying the function driver that it has become
usable.

Obviously, this can only work for CAP_NONREMOVABLE devices, but those
are exactly the ones we are worried about here. The advantage is that
the power sequencing for a particular device can then be in device
specific code and can have an arbitrarily complex in the driver without
needing the mmc code to handle all possible corner cases.

I think this is going to be the most generic solution overall, it's
going to work with other buses and it's going to allow the device to
fully power itself off while running (I don't know if it's useful for SD
but that's definitely common with some other buses).
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