enable mcp2515 and the can network drivers as modules and also add a parametrizable overlay

[msperl]

Maybe you want to help all the people who still rely on an externally compiled 3.12.X to make CAN-bus work out of the box by including the can drivers for the mcp2515.

What would be needed is:

.config:

CONFIG_CAN=m
CONFIG_CAN_RAW=m
CONFIG_CAN_BCM=m
CONFIG_CAN_GW=m
CONFIG_CAN_VCAN=m
CONFIG_CAN_DEV=m
CONFIG_CAN_CALC_BITTIMING=y
CONFIG_CAN_MCP251X=m

/boot/overlays/mcp2515-spi0.0.dts:

/*
 * Device tree overlay for mcp2515 - can0 on spi0.0
 */

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2835", "brcm,bcm2836", "brcm,bcm2708", "brcm,bcm2709";
    /* disable spi-dev for spi0.0 */
    fragment@0 {
        target = <&spi0>;
        __overlay__ {
            status = "okay";
            spidev@0{
                status = "disabled";
            };
        };
    };

    /* the interrupt pin of the can-controller */
    fragment@1 {
        target = <&gpio>;
        __overlay__ {
            can0_pins: can0_pins {
                brcm,pins = <25>;
                brcm,function = <0>; /* input */
            };
        };
    };

    /* the clock/oscillator of the can-controller */
    fragment@2 {
        target-path = "/clocks";
        __overlay__ {
            /* external oscillator of mcp2515 on SPI0.0 */
            can0_osc: can0_osc {
                compatible = "fixed-clock";
                #clock-cells = <0>;
                clock-frequency  = <16000000>;
            };
        };
    };

    /* the spi config of the can-controller itself binding everything together */
    fragment@3 {
        target = <&spi0>;
        __overlay__ {
            /* needed to avoid dtc warning */
            #address-cells = <1>;
            #size-cells = <0>;
            can0: mcp2515@0 {
                reg = <0>;
                compatible = "microchip,mcp2515";
                pinctrl-names = "default";
                pinctrl-0 = <&can0_pins>;
                spi-max-frequency = <10000000>;
                interrupt-parent = <&gpio>;
                interrupts = <25 0x2>;
                clocks = <&can0_osc>;
            };
        };
    };
    __overrides__ {
        oscillator = <&can0_osc>,"oscillator-frequency";
        spimaxfrequency = <&can0>,"spi-max-frequency:0";
        interrupt = <&can0_pins>,"brcm,pins:0",<&can0>,"interrupts:0";
    };
};

This is actually a good example of how the device tree works, as it implements a lot of things that may be needed for other overlays as well (GPIOs, Interrupts, Clocks, disable a default spi device, add a new spi device instead,...)
So it may also get used as a reference/template for other device-tree overlays.

See also raspberrypi/firmware#371

[msperl]

Pull request #868 created for this