netdev - Re: phy address in the device tree, vs auto probing

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Date:	Wed, 10 Feb 2010 12:40:57 -0600
From:	"Fleming Andy-AFLEMING" <afleming@...escale.com>
To:	"Grant Likely" <grant.likely@...retlab.ca>
Cc:	"John Linn" <John.Linn@...inx.com>,
	"devicetree-discuss" <devicetree-discuss@...ts.ozlabs.org>,
	"netdev" <netdev@...r.kernel.org>,
	"Scott Wood" <scottwood@...escale.com>
Subject: Re: phy address in the device tree, vs auto probing



On Feb 10, 2010, at 12:15, "Grant Likely" <grant.likely@...retlab.ca>  
wrote:

> On Wed, Feb 10, 2010 at 9:52 AM, John Linn <John.Linn@...inx.com>  
> wrote:
>>> -----Original Message-----
>>> From: glikely@...retlab.ca [mailto:glikely@...retlab.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@...inx.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@...00000 {
>>                        #address-cells = <1>;
>>                        #size-cells = <1>;
>>                        phy-handle = <&phy0>;
>>                        mdio {
>>                                #address-cells = <1>;
>>                                #size-cells = <0>;
>>                                phy0: phy@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@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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