On Feb 10, 2010, at 12:15, "Grant Likely" <[email protected]>
wrote:
On Wed, Feb 10, 2010 at 9:52 AM, John Linn <[email protected]>
wrote:
-----Original Message-----
From: [email protected] [mailto:[email protected]] 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 <[email protected]>
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: ether...@81000000 {
#address-cells = <1>;
#size-cells = <1>;
phy-handle = <&phy0>;
mdio {
#address-cells = <1>;
#size-cells = <0>;
phy0: p...@7 {
} ;
} ;
2. Or a special value (-1 or something not 0 - 31) in the phy
address that specifies to auto probe as illustrated below.
phy0: p...@7 {
reg = <-1>;
} ;
I don't like abusing the reg property in this way. I wonder if a new
empty property would be a better way to indicate this. Maybe
"phy-probe-address;"? It would also be important to specify in the
binding that only one phy node is allowed when phy-probe-address is
used.
I don't think it's necessary that only one phy node is there. I don't
think the of mdio layer should set policy, here. Some drivers hard
code their addresses. Some drivers assume (foolishly, I think) that
the PHYs are in order. Many assume there's only one PHY. I think the
mdio driver should set policy, so of_mdio should just allow for PHYs
to be probed. I'm actually not sure that requires any changes. Quite
possibly this just means that of_mdio is not appropriate for such a
driver. The standard PHY code supports this sort of thing.
I don't think this can reasonably be done with of_phy_connect(),
because not every driver will want the same behavior.
If you can't have your bootloader modify the tree to have addresses
(and then it can do the probe however it wants, in a board-specific
way), then you could add some board code or mdio driver code which
modifies the device tree to have addresses.
Andy
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