Subject: Re: [PATCH] [v2] netdev/phy: add MDIO bus multiplexer driven by a memory-mapped device

From: Stephen Warren <[email-masked]>
Date: Thu, 23 Aug 2012 16:54:53 -0600

On 08/23/2012 01:44 PM, Timur Tabi wrote:
> Add support for an MDIO bus multiplexer controlled by a simple memory-mapped
> device, like an FPGA.  The device must be memory-mapped and contain only
> 8-bit registers (which keeps things simple).

> +++ b/Documentation/devicetree/bindings/net/mdio-mux-mmioreg.txt

> +	/* The FPGA node */
> +	fpga: board-control@3,0 {
> +		compatible = "fsl,p5020ds-fpga", "fsl,fpga-ngpixis";
> +		reg = <3 0 0x30>;

Why not add the following here:

	#address-cells = <1>:
	#size-cells = <1>;
	ranges = <...>;

> +
> +		mdio-mux-emi2 {
> +			compatible = "mdio-mux-mmioreg", "mdio-mux";
> +			mdio-parent-bus = <&xmdio0>;
> +			#address-cells = <1>;
> +			#size-cells = <0>;
> +			reg = <9>; // BRDCFG1

Then, that'd have to be <9 1>;

That way, ...

> diff --git a/drivers/net/phy/mdio-mux-mmioreg.c b/drivers/net/phy/mdio-mux-mmioreg.c

> +static int __devinit mdio_mux_mmioreg_probe(struct platform_device *pdev)

> +	/* The MMIO device is the parent of this device */
> +	np2 = of_get_parent(np);
> +	if (!np2) {
> +		dev_err(&pdev->dev, "could not find parent MMIO device\n");
> +		return -ENODEV;
> +	}
> +
> +	ret = of_address_to_resource(np2, 0, &res);
> +	if (ret) {
> +		dev_err(&pdev->dev, "could not obtain memory map for node %s\n",
> +			np2->full_name);
> +		return ret;
> +	}
> +	s->phys = res.start;

You could simplify all that into just "of_iomap(np, 0)", and all the
address translation etc. happens entirely automatically, in the standard
fashion for DT.

The advantage here is that it completely decouples the mdio-mux-mmioreg
driver from any knowledge of its parent; you could just as easily use
this driver/binding as a device directly on some SoC bus, rather than
requiring there to be a parent device above it. After all, the mux
register might be some random standalone on-SoC register (although I
wonder if that case might not be better covered by a mdio-mux-pinctrl
driver instead; I guess it depends on how special-purpose the mux
register is).