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

From: [email-masked] (Andrew Lunn)
Date: Sun, 15 Jan 2017 20:16:32 +0100

> > What exactly is the relationship between these devices (a ascii-art tree
> > or sysfs tree output might be nice) so I can try to understand what is
> > going on here.

Hi Greg, Florian

A few diagrams and trees which might help understand what is going on.

The first diagram comes from the 2008 patch which added all this code:

            +-----------+       +-----------+
            |           | RGMII |           |
            |           +-------+           +------ 1000baseT MDI ("WAN")
            |           |       |  6-port   +------ 1000baseT MDI ("LAN1")
            |    CPU    |       |  ethernet +------ 1000baseT MDI ("LAN2")
            |           |MIImgmt|  switch   +------ 1000baseT MDI ("LAN3")
            |           +-------+  w/5 PHYs +------ 1000baseT MDI ("LAN4")
            |           |       |           |
            +-----------+       +-----------+

We have an ethernet switch and a host CPU. The switch is connected to
the CPU in two different ways. RGMII allows us to get Ethernet frames
from the CPU into the switch. MIImgmt, is the management bus normally
used for Ethernet PHYs, but Marvell switches also use it for Managing
switches.

The diagram above is the simplest setup. You can have multiple
Ethernet switches, connected together via switch ports. Each switch
has its own MIImgmt connect to the CPU, but there is only one RGMII
link.

When this code was designed back in 2008, it was decided to represent
this is a platform device, and it has a platform_data, which i have
slightly edited to keep it simple:

struct dsa_platform_data {
        /*
         * Reference to a Linux network interface that connects
         * to the root switch chip of the tree.
         */
        struct device   *netdev;

        /*
         * Info structs describing each of the switch chips
         * connected via this network interface.
         */
        int             nr_chips;
        struct dsa_chip_data    *chip;
};

This netdev is the CPU side of the RGMII interface.

Each switch has a dsa_chip_data, again edited:

struct dsa_chip_data {
        /*
         * How to access the switch configuration registers.
         */
        struct device   *host_dev;
        int             sw_addr;
...
}

The host_dev is the CPU side of the MIImgmt, and we have the address
the switch is using on the bus.

During probe of this platform device, we need to get from the
struct device *netdev to a struct net_device *dev.

So the code looks in the device net class to find the device

|   |   |   |-- f1074000.ethernet
|   |   |   |   |-- deferred_probe
|   |   |   |   |-- driver -> ../../../../../bus/platform/drivers/mvneta
|   |   |   |   |-- driver_override
|   |   |   |   |-- modalias
|   |   |   |   |-- net
|   |   |   |   |   `-- eth1
|   |   |   |   |       |-- addr_assign_type
|   |   |   |   |       |-- address
|   |   |   |   |       |-- addr_len
|   |   |   |   |       |-- broadcast
|   |   |   |   |       |-- carrier
|   |   |   |   |       |-- carrier_changes
|   |   |   |   |       |-- deferred_probe
|   |   |   |   |       |-- device -> ../../../f1074000.ethernet

and then use container_of() to get the net_device.

Similarly, the code needs to get from struct device *host_dev to a struct mii_bus *.

|   |   |   |-- f1072004.mdio
|   |   |   |   |-- deferred_probe
|   |   |   |   |-- driver -> ../../../../../bus/platform/drivers/orion-mdio
|   |   |   |   |-- driver_override
|   |   |   |   |-- mdio_bus
|   |   |   |   |   `-- f1072004.mdio-mi
|   |   |   |   |       |-- deferred_probe
|   |   |   |   |       |-- device -> ../../../f1072004.mdio

    Andrew