How to support SDIO wifi/bt in DT

[Alexandre Belloni]

On Thu, Jan 16, 2014 at 01:36:49PM +0000, Russell King - ARM Linux wrote :
> Okay, I'm hitting something of a dead end with trying to work out how to
> support this in DT.
> 
> The Wifi/BT (bcrmfmac) device has multiple connections to the host device
> - it has a SDIO connection, a UART connection, and an audio connection.
> In addition, a series of GPIOs are wired to control this device:
> 
> - Reset for bluetooth plus two additional GPIOs (unused I think)
> - Reset for Wifi plus two additional GPIOs (again, I think unused)
> - GPIO to control regulator supplying power to the Wifi/BT chip
> - GPIO to control regulator supplying power to the oscillator for
>   the Wifi/BT chip.
> 
> The Wifi/BT chip can only be detected via probing the SDIO connection, and
> only when the device has been powered up and released from reset - so we
> have to power up and reset the bcrm device before we probe via the SDIO
> bus.
> 
> While it's possible to attach the power supply for the Wifi/BT chip to the
> vmmc-supply property of the host, it's not possible to do that with the
> oscillator supply.  Neither is there any provision for manipulating the
> GPIOs to deal with the resets.
> 
> I can't find any examples of anything similar in our existing set of DT
> files, so I suspect either this is a device which no one supports on any
> DT platform, or there's some clever way to handle this.
> 
> How do other people support this in DT?  Do they hack up some platform
> specific code (which isn't nice)?  What other solutions are there to get
> around this problem?  How does this kind of thing get represented in DT?
> 
> (Don't suggest adding DT support to the bcrmfmac driver - this is a
> chicken-and-egg problem.  The driver isn't being probed at the moment
> because the device is powered down and/or held in reset, so is
> undetectable.  The kernel needs to power it up and release the reset
> so it becomes detectable.)

I hit exactly the same issue trying to support a TiWi chip on SDIO. On
my side, I used vmmc-supply to drive the reset GPIO with a fixed
regulator. The other thing needed was the clock. That one, I put it in
the board file. I guess we need a way to describe children of the SDIO
host and the host driver would be responsible to manage power supplies,
resets and clocks...

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com