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Message-ID: <fa686aa41002101120v56ef5e7cm64e5505cc7bedc1d@mail.gmail.com>
Date: Wed, 10 Feb 2010 12:20:14 -0700
From: Grant Likely <grant.likely@...retlab.ca>
To: Fleming Andy-AFLEMING <afleming@...escale.com>
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 Wed, Feb 10, 2010 at 11:40 AM, Fleming Andy-AFLEMING
<afleming@...escale.com> wrote:
>
>
> 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.
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@...00000 {
#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@...00000 {
#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.
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