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Date:   Thu, 3 Sep 2020 22:09:01 +0200
From:   Adam Rudziński <adam.rudzinski@....net.pl>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, m.felsch@...gutronix.de,
        hkallweit1@...il.com, richard.leitner@...data.com,
        zhengdejin5@...il.com, devicetree@...r.kernel.org,
        kernel@...gutronix.de, kuba@...nel.org, robh+dt@...nel.org
Subject: Re: [RFC net-next 2/2] net: phy: bcm7xxx: request and manage GPHY
 clock


W dniu 2020-09-03 o 21:35, Florian Fainelli pisze:
>
>
> On 9/3/2020 12:21 PM, Adam Rudziński wrote:
>>
>> W dniu 2020-09-03 o 19:17, Florian Fainelli pisze:
>>>
>>>
>>> On 9/3/2020 10:13 AM, Adam Rudziński wrote:
>>>>
>>>> W dniu 2020-09-03 o 17:21, Florian Fainelli pisze:
>>>>>
>>>>>
>>>>> On 9/2/2020 11:00 PM, Adam Rudziński wrote:
>>>>>>
>>>>>> W dniu 2020-09-03 o 04:13, Florian Fainelli pisze:
>>>>>>>
>>>>>>>
>>>>>>> On 9/2/2020 3:20 PM, Andrew Lunn wrote:
>>>>>>>>> +    priv->clk = devm_clk_get_optional(&phydev->mdio.dev, 
>>>>>>>>> "sw_gphy");
>>>>>>>>> +    if (IS_ERR(priv->clk))
>>>>>>>>> +        return PTR_ERR(priv->clk);
>>>>>>>>> +
>>>>>>>>> +    /* To get there, the mdiobus registration logic already 
>>>>>>>>> enabled our
>>>>>>>>> +     * clock otherwise we would not have probed this device 
>>>>>>>>> since we would
>>>>>>>>> +     * not be able to read its ID. To avoid artificially 
>>>>>>>>> bumping up the
>>>>>>>>> +     * clock reference count, only do the clock enable from a 
>>>>>>>>> phy_remove ->
>>>>>>>>> +     * phy_probe path (driver unbind, then rebind).
>>>>>>>>> +     */
>>>>>>>>> +    if (!__clk_is_enabled(priv->clk))
>>>>>>>>> +        ret = clk_prepare_enable(priv->clk);
>>>>>>>>
>>>>>>>> This i don't get. The clock subsystem does reference counting. 
>>>>>>>> So what
>>>>>>>> i would expect to happen is that during scanning of the bus, 
>>>>>>>> phylib
>>>>>>>> enables the clock and keeps it enabled until after probe. To keep
>>>>>>>> things balanced, phylib would disable the clock after probe.
>>>>>>>
>>>>>>> That would be fine, although it assumes that the individual PHY 
>>>>>>> drivers have obtained the clocks and called 
>>>>>>> clk_prepare_enable(), which is a fair assumption I suppose.
>>>>>>>
>>>>>>>>
>>>>>>>> If the driver wants the clock enabled all the time, it can 
>>>>>>>> enable it
>>>>>>>> in the probe method. The common clock framework will then have two
>>>>>>>> reference counts for the clock, so that when the probe exists, and
>>>>>>>> phylib disables the clock, the CCF keeps the clock ticking. The 
>>>>>>>> PHY
>>>>>>>> driver can then disable the clock in .remove.
>>>>>>>
>>>>>>> But then the lowest count you will have is 1, which will lead to 
>>>>>>> the clock being left on despite having unbound the PHY driver 
>>>>>>> from the device (->remove was called). This does not allow 
>>>>>>> saving any power unfortunately.
>>>>>>>
>>>>>>>>
>>>>>>>> There are some PHYs which will enumerate with the clock 
>>>>>>>> disabled. They
>>>>>>>> only need it ticking for packet transfer. Such PHY drivers can 
>>>>>>>> enable
>>>>>>>> the clock only when needed in order to save some power when the
>>>>>>>> interface is administratively down.
>>>>>>>
>>>>>>> Then the best approach would be for the OF scanning code to 
>>>>>>> enable all clocks reference by the Ethernet PHY node (like it 
>>>>>>> does in the proposed patch), since there is no knowledge of 
>>>>>>> which clock is necessary and all must be assumed to be critical 
>>>>>>> for MDIO bus scanning. Right before drv->probe() we drop all 
>>>>>>> resources reference counts, and from there on ->probe() is 
>>>>>>> assumed to manage the necessary clocks.
>>>>>>>
>>>>>>> It looks like another solution may be to use the assigned-clocks 
>>>>>>> property which will take care of assigning clock references to 
>>>>>>> devices and having those applied as soon as the clock provider 
>>>>>>> is available.
>>>>>>
>>>>>> Hi Guys,
>>>>>>
>>>>>> I've just realized that a PHY may also have a reset signal 
>>>>>> connected. The reset signal may be controlled by the dedicated 
>>>>>> peripheral or by GPIO.
>>>>>
>>>>> There is already support for such a thing within 
>>>>> drivers/net/phy/mdio_bus.c though it assumes we could bind the PHY 
>>>>> device to its driver already.
>>>>>
>>>>>>
>>>>>> In general terms, there might be a set of control signals needed 
>>>>>> to enable the PHY. It seems that the clock and the reset would be 
>>>>>> the typical useful options.
>>>>>>
>>>>>> Going further with my imagination of how evil the hardware design 
>>>>>> could be, in general the signals for the PHY may have some 
>>>>>> relations to other control signals.
>>>>>>
>>>>>> I think that from the software point of view this comes down to 
>>>>>> assumption that the PHY is to be controlled "driver only knows how".
>>>>>
>>>>> That is all well and good as long as we can actually bind the PHY 
>>>>> device which its driver, and right now this means that we either 
>>>>> have:
>>>>>
>>>>> - a compatible string in Device Tree which is of the form 
>>>>> ethernet-phy-id%4x.%4x (see of_get_phy_id) which means that we 
>>>>> *know* already which PHY we have and we avoid doing reads of 
>>>>> MII_PHYSID1 and MII_PHYSID2. This is a Linux implementation detail 
>>>>> that should not have to be known to systems designer IMHO
>>>>>
>>>>> - a successful read of MII_PHYSID1 and MII_PHYSID2 (or an 
>>>>> equivalent for the clause 45 PHYs) that allows us to know what PHY 
>>>>> device we have, which is something that needs to happen eventually.
>>>>>
>>>>> The problem is when there are essential resources such as clocks, 
>>>>> regulators, reset signals that must be enabled, respectively 
>>>>> de-asserted in order for a successful MDIO read of MII_PHYSID1 and 
>>>>> MII_PHYSID2 to succeed.
>>>>>
>>>>> There is no driver involvement at that stage because we have no 
>>>>> phy_device to bind it to *yet*. Depending on what we read from 
>>>>> MII_PHYSID1/PHY_ID2 we will either successfully bind to the 
>>>>> Generic PHY driver (assuming we did not read all Fs) or not and we 
>>>>> will return -ENODEV and then it is game over.
>>>>>
>>>>> This is the chicken and egg problem that this patch series is 
>>>>> addressing, for clocks, because we can retrieve clock devices with 
>>>>> just a device_node reference.
>>>>
>>>> I have an impression that here the effort goes in the wrong 
>>>> direction. If I understand correctly, the goal is to have the 
>>>> kernel find out what will the driver need to use the phy. But, the 
>>>> kernel anyway calls a probe function of the driver, doesn't it? To 
>>>> me it looks as if you were trying to do something that the driver 
>>>> will/would/might do later, and possibly "undo" it in the meantime. 
>>>> In this regard, this becomes kind of a workaround, not solution of 
>>>> the problem. Also, having taken a glance at your previous messages, 
>>>> I can tell that this is all becoming even more complex.
>>>
>>> What is the problem according to you, and what would an acceptable 
>>> solution look like then?
>>>
>>>>
>>>> I think that the effort should be to allow any node in the device 
>>>> tree to take care about its child nodes by itself, and just 
>>>> "report" to the kernel, or "install" in the kernel whatever is 
>>>> necessary, but without any initiative of the kernel. Let the 
>>>> drivers be as complicated as necessary, not the kernel.
>>>
>>> The Device Tree representation is already correct in that it lists 
>>> clocks/regulators/reset signals that are needed by the PHY. The 
>>> problem is its implementation with the Linux device driver model. 
>>> Please read again what I wrote.
>>
>> The problem which I had, was that kernel was unable to read ID of 
>> second PHY, because it needed to have the clock enabled, and during 
>> probing it still didn't have. I don't know what problems are 
>> addressed by the discussed patches - only the same one, or if 
>> something else too.
>
> That is precisely the problem being addressed here, an essential clock 
> to the PHY must be enabled for the PHY to be accessible on the MDIO bus.
>
>> In my solution, of my problem, which now works well for me, instead 
>> of teaching kernel any new tricks, I've taught the kernel to listen 
>> to the driver, by adding a function allowing the driver to register 
>> its PHY on the MDIO bus at any time. Then, each driver instance (for 
>> a particular interface) configures whatever is necessary, finds its 
>> PHY when probing the shared MDIO bus, and tells the kernel(?) to add 
>> it to the shared MDIO bus. Since each one does that, when the time 
>> comes, all PHYs are known and all interfaces are up.
>
> Except this is bending the Linux driver model backwards, a driver does 
> not register devices, a bus does, and then it binds the driver to that 
> device object. For the bus to scan a hardware device, said hardware 
> device must be in a functional state to be discovered.

OK. Probably I'd better just have said that all the mentioned actions 
were a consequence of calling the driver's probe function (fec_probe() 
in the example of IMX FEC driver) and were not relying on any 
pre-configured features. My description in fact was very... imprecise.

>> This is just an example. It doesn't need the kernel to mimic the 
>> driver. But, it requires a bit different structure of the device 
>> tree, and I guess I'm not aware of tons of reasons for which it's not 
>> "the good way". Sorry, I can't be more constructive here, I don't 
>> have that much experience with the kernel. The idea is simple: the 
>> driver does the job, not the kernel.
>> You know much better than me how Linux works, so you decide if this 
>> makes sense, or not, and what does this actually mean in the context 
>> of the system.

Best regards,
Adam


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