Re: [PATCH net-next v3 06/10] net: dsa: Migrate to device_find_class()

[Florian Fainelli]

On 01/19/2017 08:51 AM, Russell King - ARM Linux wrote:
> (This is mainly for Greg's benefit to help him understand the issue.)
> 
> I think the diagram you gave initially made this confusing, as it
> talks about a CPU(sic) producing the "RGMII" and "MII-MGMT".
> 
> Let's instead show a better representation that hopefully helps Greg
> understand networking. :)
> 
> 
>   CPU
> System <-B->  Ethernet controller <-P-> } PHY <---> network cable
>                                         } - - - - - - - or - - - - - - -
>                   MDIO bus -------M---> } Switch <-P-> PHYs <--> network
>                                               `----M----^        cables
> 
> 'B' can be an on-SoC bus or something like PCI.
> 
> 'P' are the high-speed connectivity between the ethernet controller and
> PHY which carries the packet data.  It has no addressing, it's a point
> to point link.  RGMII is just one wiring example, there are many
> different interfaces there (SGMII, AUI, XAUI, XGMII to name a few.)
> 
> 'M' are the MDIO bus, which is the bus by which ethernet PHYs and
> switches can be identified and controlled.

So MDIO is one possible bus type to connect an Ethernet switch (and
PHYs) to a the System, but is not necessarily the only one. You have
existing devices out there with on-chip integrated switches (platform),
SPI/GPIO/I2C/PCI(e).

> 
> The MDIO bus has a bus_type, has host drivers which are sometimes
> part of the ethernet controller, but can also be stand-alone devices
> shared between multiple ethernet controllers.
> 
> PHYs are a kind of MDIO device which are members of the MDIO bus
> type.  Each PHY (and switch) has a numerical address, and identifying
> numbers within its register set which identifies the manufacturer
> and device type.  We have device_driver objects for these.
> 
> Expanding the above diagram to make it (hopefully) even clearer,
> we can have this classic setup:
> 
>   CPU
> System <-B-> Ethernet controller <-P-> PHY <---> network cable
>                  MDIO bus -------M------^
> 
> Or, in the case of two DSA switches attached to an Ethernet controller:
> 
>                                      |~~~~~~~~|
> System <-B-> Ethernet controller <-P-> Switch <-P-> PHY1 <--> network cable
>                  MDIO bus ----+--M--->   1    <-P-> PHY2 <--> network cable
>                               |      |        ...    |
>                               |      |        <-P-> PHYn <--> network cable
>                               |      |....^...|      |
>                               |           |  `---M---'
>                               |           P
>                               |           |
>                               |      |~~~~v~~~|
>                               `------> Switch <-P-> PHY1 <--> network cable
>                                      |   2    ...    |
>                                      |        <-P-> PHYn <--> network cable
>                                      |........|      |
>                                              `---M---'
> 
> The problem that the DSA guys are trying to deal with is how to
> represent the link between the DSA switches (which are devices
> sitting off their controlling bus - the MDIO bus) and the ethernet
> controller associated with that collection of devices, be it a
> switch or PHY.
> 
> Merely changing the parent/child relationships to try and solve
> one issue just creates exactly the same problem elsewhere.

Exactly!

> 
> So, I hope with these diagrams, you can see that trying to make
> the ethernet controller a child device of the DSA switches
> means that (eg) it's no longer a PCI device, which is rather
> absurd, especially when considering that what happens to the
> right of the ethernet controller in the diagrams above is
> normally external chips to the SoC or ethernet device.
> 

Indeed.

Back to the actual code that triggered this discussion, the whole
purpose is just a safeguard. Given a device reference, we can assume
that it is indeed the backing device for a net_device, and we could do a
to_net_device() right away (and crash if someone did not write correct
platform_data structures), or, by walking the device tree (the device
driver model one) we can make sure it does belong in the proper class
and this is indeed what we think it is.

HTH
-- 
Florian