linux-kernel - Re: [PATCH v4 net-next] mdio_bus: Issue GPIO RESET to PHYs.

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Message-ID: <5954130c-c08c-1555-4d34-83307ed68d92@metafoo.de>
Date:   Tue, 25 Apr 2017 18:22:25 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Roger Quadros <rogerq@...com>, Andrew Lunn <andrew@...n.ch>
Cc:     davem@...emloft.net, Florian Fainelli <f.fainelli@...il.com>,
        tony@...mide.com, nsekhar@...com, jsarha@...com,
        netdev@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 net-next] mdio_bus: Issue GPIO RESET to PHYs.

On 04/24/2017 11:04 AM, Roger Quadros wrote:
> On 24/04/17 02:35, Andrew Lunn wrote:
>> On Fri, Apr 21, 2017 at 03:31:09PM +0200, Lars-Peter Clausen wrote:
>>> On 04/21/2017 03:15 PM, Roger Quadros wrote:
>>>> diff --git a/Documentation/devicetree/bindings/net/mdio.txt b/Documentation/devicetree/bindings/net/mdio.txt
>>>> new file mode 100644
>>>> index 0000000..4ffbbac
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/net/mdio.txt
>>>> @@ -0,0 +1,33 @@
>>>> +Common MDIO bus properties.
>>>> +
>>>> +These are generic properties that can apply to any MDIO bus.
>>>> +
>>>> +Optional properties:
>>>> +- reset-gpios: List of one or more GPIOs that control the RESET lines
>>>> +  of the PHYs on that MDIO bus.
>>>> +- reset-delay-us: RESET pulse width in microseconds as per PHY datasheet.
>>>> +
>>>> +A list of child nodes, one per device on the bus is expected. These
>>>> +should follow the generic phy.txt, or a device specific binding document.
>>>> +
>>>> +Example :
>>>> +This example shows these optional properties, plus other properties
>>>> +required for the TI Davinci MDIO driver.
>>>> +
>>>> +	davinci_mdio: ethernet@...c030000 {
>>>> +		compatible = "ti,davinci_mdio";
>>>> +		reg = <0x5c030000 0x1000>;
>>>> +		#address-cells = <1>;
>>>> +		#size-cells = <0>;
>>>> +
>>>> +		reset-gpios = <&gpio2 5 GPIO_ACTIVE_LOW>;
>>>> +		reset-delay-us = <2>;   /* PHY datasheet states 1us min */
>>>
>>> If this is the reset line of the PHY shouldn't it be a property of the PHY
>>> node rather than of the MDIO controller node (which might have a reset on
>>> its own)?
>>>> +
>>>> +		ethphy0: ethernet-phy@1 {
>>>> +			reg = <1>;
>>>> +		};
>>>> +
>>>> +		ethphy1: ethernet-phy@3 {
>>>> +			reg = <3>;
>>>> +		};
>>
>> Hi Lars-Peter
>>
>> We discussed this when the first proposal was made. There are two
>> cases, to consider.
>>
>> 1) Here, one GPIO line resets all PHYs on the same MDIO bus. In this
>> example, two PHYs.
>>
>> 2) There is one GPIO line per PHY. That is a separate case, and as you
>> say, the reset line should probably be considered a PHY property, not
>> an MDIO property. However, it can be messy, since in order to probe
>> the MDIO bus, you probably need to take the PHY out of reset.
>>

But the DT binding documentation says something else "List of one or more
GPIOs that control the RESET lines of the PHYs on that MDIO bus".

>> Anyway, this patch addresses the first case, so should be accepted. If
>> anybody wants to address the second case, they are free to do so.

I think we all know that that's not going to happen. Once there is a working
kludge there is no incentive to do a proper implementation anymore.


> Thanks for the explanation Andrew.
> 
> For the second case, even if the RESET GPIO property is specified
> in the PHY node, the RESET *will* have to be done by the MDIO bus driver
> else the PHY might not be probed at all.

I'm not arguing with that, just that the hardware description should be
truthful to the hardware topology and not to the software topology, i.e. the
implementation details of the Linux kernel in this case. Reset GPIOs are not
the only resource that is connected to the PHY that needs to be enabled
before they can be enumerated. E.g. clocks and regulators fall into the same
realm. And while you might argue that with a on-SoC phy controller node
there wont be any conflicts in regard to the reset-gpios property, this not
so very true for the clocks property.

And MDIO is not really special in this regard, other discoverable buses
(like USB, SDIO, ULPI) have the very same issue. Having a standardized
binding approach where the resources are declared as part as the child child
is preferable in my opinion.

> 
> Whether we need additional code to just to make the DT look prettier is
> questionable and if required can come as a separate patch.

Unfortunately not, once it is merged it can't be changed anymore.