linux-kernel - [DT Question] "simple-mfd" DT binding

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Date:   Mon, 22 May 2017 10:29:26 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Lee Jones <lee.jones@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     masahiroy@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>
Subject: [DT Question] "simple-mfd" DT binding

Hi DT experts,

I have a question about "simple-mfd".

Documentation/devicetree/bindings/mfd/mfd.txt says as allows:

----------------------8<---------------------
Optional properties:

- compatible : "simple-mfd" - this signifies that the operating system should
  consider all subnodes of the MFD device as separate devices akin to how
  "simple-bus" indicates when to see subnodes as children for a simple
  memory-mapped bus.   <snip>
----------------------8<---------------------

I'd like to be sure about the statement above.

Does this mean, "simple-bus" and "simple-mfd" are technically interchangeable?

If so, I thought the example some lines below is questionable.

---------------------8<---------------------------
Example:

foo@...0 {
        compatible = "syscon", "simple-mfd";
        reg = <0x01000 0x1000>;

        led@...0 {
                compatible = "register-bit-led";
                offset = <0x08>;
                mask = <0x01>;
                label = "myled";
                default-state = "on";
        };
};
---------------------8<---------------------------

Because "simple-bus" indicates that child nodes are
simply memory mapped, but the node "register-bit-led"
can not be memory-mapped.
So, "simple-mfd" can not be replaced "simple-bus" here.

arch/arm/boot/dts/arm-realview-pb1176.dts is a real example.

If I replace "simple-mfd" with "simple-bus",
DTC warns "empty reg/ranges property".

$ sed -i  -e 's/simple-mfd/simple-bus/'
arch/arm/boot/dts/arm-realview-pb1176.dts
$ make -s ARCH=arm defconfig
$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- W=1  arm-realview-pb1176.dtb
   ...
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...0 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...1 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...2 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...3 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...4 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...5 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...6 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/led@...7 missing or empty reg/ranges
property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/osc0@0c missing or empty reg/ranges property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/osc1@10 missing or empty reg/ranges property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/osc2@14 missing or empty reg/ranges property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/osc3@18 missing or empty reg/ranges property
arch/arm/boot/dts/arm-realview-pb1176.dtb: Warning (simple_bus_reg):
Node /soc/syscon@...00000/osc4@1c missing or empty reg/ranges property

-- 
Best Regards
Masahiro Yamada