phy address in the device tree, vs auto probing

[Grant Likely]

On Wed, Feb 10, 2010 at 11:40 AM, Fleming Andy-AFLEMING
<afleming at freescale.com> wrote:
>
>
> On Feb 10, 2010, at 12:15, "Grant Likely" <grant.likely at secretlab.ca> wrote:
>
>> On Wed, Feb 10, 2010 at 9:52 AM, John Linn <John.Linn at xilinx.com> wrote:
>>>>
>>>> -----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.
>>>                               phy0: phy at 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.

That still doesn't solve the problem of matching PHYs to MACs.

Consider this example:  2 MACs, 2 PHYs.  mac_a--> phy_a and mac_b -->
phy_b.  Both phys on the same mdio bus, described thus:

               eth_a: ethernet at 81000000 {
                       #address-cells = <1>;
                       #size-cells = <1>;
                       phy-handle = <&phy_a>;
                       mdio {
                               #address-cells = <1>;
                               #size-cells = <0>;
                               phy_a: phy_a {
                               } ;
                               phy_b: phy_b {
                               } ;
                       } ;
               } ;
               eth_b: ethernet at 82000000 {
                       #address-cells = <1>;
                       #size-cells = <1>;
                       phy-handle = <&phy_b>;
               } ;

In this example, the kernel knows it has two phys, and probing
confirms this (say at phy addresses 3 and 7).  How does the kernel
know which address phy_a responds to?

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.