Re: PHY reset handling during DT parsing

[Florian Fainelli]

On 7/7/2020 10:18 AM, Rob Herring wrote:
> On Tue, Jul 7, 2020 at 10:39 AM Florian Fainelli <f.fainelli@xxxxxxxxx> wrote:
>>
>>
>>
>> On 7/7/2020 7:54 AM, Maxime Ripard wrote:
>>> Hi Andrew,
>>>
>>> On Tue, Jul 07, 2020 at 04:19:18PM +0200, Andrew Lunn wrote:
>>>> On Mon, Jul 06, 2020 at 08:13:31PM +0200, Maxime Ripard wrote:
>>>>> I came across an issue today on an Allwinner board, but I believe it's a
>>>>> core issue.
>>>>>
>>>>> That board is using the stmac driver together with a phy that happens to
>>>>> have a reset GPIO, except that that GPIO will never be claimed, and the
>>>>> PHY will thus never work.
>>>>>
>>>>> You can find an example of such a board here:
>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/sun6i-a31-hummingbird.dts#n195
>>>>>
>>>>> It looks like when of_mdiobus_register() will parse the DT, it will then
>>>>> call of_mdiobus_register_phy() for each PHY it encounters [1].
>>>>> of_mdiobus_register_phy() will then if the phy doesn't have an
>>>>> ethernet-phy-id* compatible call get_phy_device() [2], and will later on
>>>>> call phy_register_device [3].
>>>>>
>>>>> get_phy_device() will then call get_phy_id() [4], that will try to
>>>>> access the PHY through the MDIO bus [5].
>>>>>
>>>>> The code that deals with the PHY reset line / GPIO is however only done
>>>>> in mdiobus_device_register, called through phy_device_register. Since
>>>>> this is happening way after the call to get_phy_device, our PHY might
>>>>> still very well be in reset if the bootloader hasn't put it out of reset
>>>>> and left it there.
>>>>
>>>> Hi Maxime
>>>>
>>>> If you look at the history of this code,
>>>>
>>>> commit bafbdd527d569c8200521f2f7579f65a044271be
>>>> Author: Sergei Shtylyov <sergei.shtylyov@xxxxxxxxxxxxxxxxxx>
>>>> Date:   Mon Dec 4 13:35:05 2017 +0100
>>>>
>>>>     phylib: Add device reset GPIO support
>>>>
>>>> you will see there is an assumption the PHY can be detected while in
>>>> reset. The reset was originally handled inside the at803x PHY driver
>>>> probe function, before it got moved into the core.
>>>>
>>>> What you are asking for it reasonable, but you have some history to
>>>> deal with, changing some assumptions as to what the reset is all
>>>> about.
>>>
>>> Thanks for the pointer.
>>>
>>> It looks to me from the commit log that the assumption was that a
>>> bootloader could leave the PHY into reset though?
>>>
>>> It starts with:
>>>
>>>> The PHY devices sometimes do have their reset signal (maybe even power
>>>> supply?) tied to some GPIO and sometimes it also does happen that a
>>>> boot loader does not leave it deasserted.
>>>
>>> This is exactly the case I was discussing. The bootloader hasn't used
>>> the PHY, and thus the PHY reset signal is still asserted?
>>
>> The current solution to this problem would be to have a reset property
>> specified for the MDIO bus controller node such that the reset would be
>> de-asserted during mdiobus_register() and prior to scanning the MDIO bus.
> 
> Unless the reset controls all the devices on the mdio bus, the
> controller node is not the right place. The core could look into the
> child nodes, but this is just one possible property.

Both are defined and supported (with the caveat mentioned below).

> 
>> This has the nice property that for a 1:1 mapping between MDIO bus
>> controller and device, it works (albeit maybe not being accurately
>> describing hardware), but if you have multiple MDIO/PHY devices sitting
>> on the bus, they might each have their own reset control and while we
>> would attempt to manage that reset line from that call path:
>>
>> mdiobus_scan()
>>  -> phy_device_register()
>>     -> mdiobus_register_device()
>>
>> it would be too late because there is a preceding get_phy_device() which
>> attempts to read the PHY device's OUI and it would fail if the PHY is
>> still held in reset.
>>
>> We have had a similar discussion before with regulators:
>>
>> http://archive.lwn.net:8080/devicetree/20200622093744.13685-1-brgl@xxxxxxxx/
>>
>> and so far we do not really have a good solution to this problem either.
>>
>> For your specific use case with resets you could do a couple of things:
>>
>> - if there is only one PHY on the MDIO bus you can describe the reset
>> line to be within the MDIO bus controller node (explained above), this
>> is not great but it is not necessarily too far from reality either
>>
>> - if you know the PHY OUI, you can put it as a compatible string in the
>> form: ethernet-phy-id%4x.%4x and that will avoid the MDIO bus layer to
>> try to read from the PHY but instead match it with a driver and create a
>> phy_device instance right away
> 
> The h/w is simply not discoverable, so it needs a compatible string.

It is discoverable if you have the various resources (regulator(s),
clock(s) and reset(s) managed ahead of issuing the discovery which is
really the crux of the problem here.

> 
>> I think we are open to a "pre probe" model where it is possible to have
>> a phy_device instance created that would allow reset controller or
>> regulator (or clocks, too) to be usable with a struct device reference,
>> yet make no HW access until all of these resources have been enabled.
> 
> I think this is needed for the kernel's driver model in general. It's
> not an MDIO specific problem.

Oh absolutely not, PCI and USB have the same issue, it is not clear to
me how to tackle this yet though.
-- 
Florian