Subject: Re: [RFC PATCH 0/3] Enable connecting DSA-based switch to the USB RMII interface.

From: Florian Fainelli <[email-masked]>
Date: Tue, 21 Apr 2015 10:18:31 -0700

On 21/04/15 05:47, Andrew Lunn wrote:
> Hi Jan
> 
> Interesting work, but i think the architecture is wrong.
> 
> DSA needs an Ethernet device, an MDIO bus, and information about ports
> on the switch. 

That requirement is completely artificial as it is today, and just comes
from arbitrary limitations imposed in the initial DSA design, something
that I am still trying to get away from.

> The MDIO bus and the Ethernet need no knowledge of
> DSA. So putting your DSA configuration code in the MDIO driver is
> wrong.

I agree with that.

> 
> The problem you have is where the put the configuration data. There
> are the currently two choices, using a platform driver, which you can
> find some examples of in arch/arm/mach-orion5x, or via device tree. Or
> you need a new method.
> 
> Part of your problem is hotplug, since you have a USB device, and no
> stable names for the ethernet device nor the MDIO device. Your
> hardware is not fixed, you could hang any switch off the USB
> device. So it does sound like you need a user space API.
> 
> I would however say that sysfs is the wrong API. The linux network
> stack uses netlink for most configuration activities. So i would
> suggest adding a netlink binding to DSA, and place the code in
> net/dsa/, not within an MDIO driver.

I suppose we could do that, but that sounds like a pretty radical change
in how DSA is currently configured (that is statically at boot time),
part in order to allow booting from DSA-enabled network devices (e.g:
nfsroot).

> 
> Device tree overlays might be a solution, if you can dynamically load
> a blob as part of a USB hotplug event. What makes it easier is that
> both the Ethernet device and MDIO bus are on the same USB device, so
> all your phandles are within the blob.
> 
> What is your long term goal? Is this just a development tool? Are you
> thinking of making a product which integrates both the switch and the
> USB ethernet onto a USB dongle? This could also change the
> architecture, since it makes the configuration more fixed.

My goal in reworking this weird DSA device/driver model is that you
could just register your switch devices as an enhanced
phy_driver/spi_driver/pci_driver etc..., such that libphy-ready drivers
could just take advantage of that when they scan/detect their MDIO buses
and find a switch. We are not quite there yet, but some help could be
welcome, here are the WIP patches (tested with platform_driver only so far):

https://github.com/ffainelli/linux/tree/dsa-model-b53
-- 
Florian