phy address in the device tree, vs auto probing

[John Linn]

> -----Original Message-----
> From: glikely at secretlab.ca [mailto:glikely at secretlab.ca] On Behalf Of Grant Likely
> Sent: Wednesday, February 10, 2010 9:44 AM
> To: John Linn; devicetree-discuss; netdev
> Subject: Re: phy address in the device tree, vs auto probing
> 
> (cc'ing devicetree-discuss and netdev mailing lists)
> 
> On Tue, Feb 9, 2010 at 4:23 PM, John Linn <John.Linn at xilinx.com> wrote:
> > Hi Grant,
> >
> > I notice that the OF driver for the mdio bus is not doing auto probing.
> >
> > As we start putting in the phy layer in the emac drivers, the device
> > trees tend to have the phy address in them, but we're not sure we really
> > like that.
> >
> > We really think that being able to let the kernel find the phy address
> > is a big benefit, otherwise this is one other piece of info the user has
> > to know and get right.
> >
> > Am I missing something here?
> 
> No, you're not really missing something, but there is an inherent
> complexity in what you're wanting to do.  Like i2c, MDIO is one of
> those busses that is hard to probe reliable.  Some PHYs respond on
> more than one address, and there is no way to determine which MAC a
> PHY is wired up to.  Many PHYs can live on a single MDIO bus.  MACs
> with their own MDIO busses may still get wired to a PHY on a different
> bus.
> 
> In the simple case where there is a one:one:one relationship between
> MAC, MDIO bus and PHY, then it should be okay to probe the PHY,
> correct?  The question then must be asked; how does the kernel
> determine that it can use the simple case?  Nobody has yet defined a
> way to describe that in the device tree; mostly because nobody has
> needed to yet.
> 
> So, it is possible to do what you want, but you need a way to
> *explicitly* ask for that behaviour.  ie, some way to indicate in a
> MAC node which MDIO bus the phy is on, and that the phy needs to be
> probed for.  I think this should only be an option when the MDIO bus
> has only one PHY.  Come up with a proposal and post it to the
> devicetree-discuss mailing list.

Here's a couple ideas. See what everyone thinks as I'm not stuck on either.

Thanks,
John

1. What if we just don't specific a phy address with a reg property which would specify to auto probe it and find the phy as illustrated below?


		Ethernet_MAC: ethernet at 81000000 {
			#address-cells = <1>;
			#size-cells = <1>;
			phy-handle = <&phy0>; 
			mdio {
				#address-cells = <1>;
				#size-cells = <0>;
				phy0: phy at 7 {
				} ; 
			} ;

2. Or a special value (-1 or something not 0 - 31) in the phy address that specifies to auto probe as illustrated below.

		Ethernet_MAC: ethernet at 81000000 {
			#address-cells = <1>;
			#size-cells = <1>;
			phy-handle = <&phy0>; 
			mdio {
				#address-cells = <1>;
				#size-cells = <0>;
				phy0: phy at 7 {
					reg = <-1>;
				} ; 
			} ;

> 
> g.
> 
> >
> > Thanks,
> > John
> >
> >
> > int of_mdiobus_register(struct mii_bus *mdio, struct device_node *np)
> > {
> >        struct phy_device *phy;
> >        struct device_node *child;
> >        int rc, i;
> >
> >        /* Mask out all PHYs from auto probing.  Instead the PHYs listed
> > in
> >         * the device tree are populated after the bus has been
> > registered */
> >        mdio->phy_mask = ~0;
> >
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> >
> >
> 
> 
> 
> --
> Grant Likely, B.Sc., P.Eng.
> Secret Lab Technologies Ltd.


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