RFC: representing sdio devices oob interrupt, clks, etc. in device tree

[Hans de Goede]

Hi All,

Arend asked me to test these 2 patches for adding devicetree support to brcmfmac sdio devices:
"dt: bindings: add bindings for Broadcom bcm43xx sdio devices"
"brcmfmac: add device tree support for SDIO devices"
https://groups.google.com/forum/#!msg/linux-sunxi/Zph6zDTnAcw/_-wOO-gnIuQJ

Getting this to do anything at all meant also adding this patch:

"mmc: Add SDIO function devicetree subnode parsing"
https://lkml.org/lkml/2014/4/3/522

So that the card / sdio-func devices would actually get their of_node
pointer set to the child node under the mmc controller describing
the sdio wifi module.

But that does not solve the entire problem. Listing the oob-interrupt info
in the child node works fine. But listing the brcm,wlan-supply gpio does
not. Since these sdio wifi modules are typically soldered onto the
board, there mmc controller device tree node has non-removable; set,
so the mmc subsystem will try to initialize the sdio device as soon
as the mmc driver loads, and if that fails will never look at it again.

So we need to have the brcm,wlan-supply gpio driven high *before* the mmc
host driver initializes. In case of the brcm,wlan-supply property, this
is actually a problem which we've already solved in a number of dts files
for allwinner boards, simply create a fixed-regulator with an enable/disable
gpio, and set that as vmmc-supply for the mmc-host for the sdio wifi.

However the "brcmfmac: add device tree support for SDIO devices" patch
also adds support for 2 devicetree controlled clocks. Assuming these need
to be ungated before the sdio module will respond to mmc commands, we have
the same problem.

I've been thinking a bit about this, and it is a non trivial problem since
sdio devices are normally instantiated when probed, unlike ie spi devices
which are instantiated from devicetree.

Adding device tree instantiation of sdio devices seems like a bad idea, as
then we get 2 vastly different device instantiation paths. Still we need some
way to get power and clks setup before the mmc host initializes.

Therefor I would like to suggest to add a number of standard properties to
mmc-bus child nodes. Making the dts for an mmc host with an sdio device which
needs clks setup before it will work look like this:

                mmc3: mmc at 01c12000 {
                        #address-cells = <1>;
                        #size-cells = <0>;
                        pinctrl-names = "default";
                        pinctrl-0 = <&mmc3_pins_a>;
                        vmmc-supply = <&reg_vmmc3>;
                        bus-width = <4>;
                        non-removable;
                        status = "okay";

                        brcmf: bcrmf at 0 {
                                reg = <0>;
                                compatible = "brcm,bcm43xx-fmac";
				clocks = <&clk_out_a>, <&clk_out_b>;
				clock-frequency = <32768>, <26000000>;
                                interrupt-parent = <&pio>;
                                interrupts = <10 4>; /* PH10 / EINT10 */
                                interrupt-names = "host-wake";
                                status = "okay";
                        };
                };

Where the "clocks" and "clock-frequency" attributes would be something
which we standardize in Documentation/devicetree/bindings/mmc/mmc-bus.txt

These standard properties would *not* be used by the driver for the
childnode device, so as to avoid the chicken and egg problem of that driver
needing to enable clks, and it only getting bound to the device after
the device has been discovered which requires those clocks.

Instead these properties would be parsed by mmc_of_parse, and we will
get enabled automatically by mmc_add_host before doing any probing.

If the optional clock-frequency property is also set, then mmc_add_host
will not only enable the clks but also set them to the specified
frequency.

Note I've no boards which actually need the clocks, all 3 boards with
sdio wifi which I've use a 26MHz crystal directly attached to the module,
so once I get the oob interrupt working (this needs some work on
the allwinner side) we can merge Arend's patches with the clock bits
dropped for now.

Still I want to get a discussion going on this, so that when the time
comes that we do need it we know how to deal with this, or ideally
already have code in place to deal with it.

Regards,

Hans