Re: Non-enumerable devices on USB and other enumerable buses

From: Alan Stern
Date: Sun Aug 11 2013 - 21:53:17 EST

On Sun, 11 Aug 2013, Mark Brown wrote:

> Looking at the enumerable buses in the kernel I don't see any which have
> real support for any kind of registration of devices prior to their
> enumeration. Similarly currently all the DT bindings in the kernel I've
> been able to notice cover only non-enumerable buses. This generally
> makes sense where enumerable buses are used in standard fashions since
> the binding would be at best redundant and at worst inaccurate. However
> there are often corner cases in embedded systems where hardware has been
> hooked up outside of the normal enumeration mechanisms, for example with
> software power control or extra signals wired.
> One example that's bugging me right now is that on the Insignal Arndale
> platform there's a USB hub connected to one of the USB ports on the SoC
> (not as a PHY, it seems we also need the internal PHY running to talk to
> the device). The hub needs to be "plugged" into the SoC after the SoC
> USB controller has started with some GPIOs so we need to tell the system
> that the hub exists and needs to be synchronised with the USB controller.

On the surface, this seems like a particularly simple case. Why wait
until the SoC's USB controller has started? Why not "plug in" the hub
via the GPIOs right from the beginning?

> Another case that's going to be problematic once it's in mainline is
> Slimbus - this is a bus used in some embedded audio subsystems which is
> enumerable in a similar manner to USB but where the devices on the bus
> are normally powered up only on demand (causing them to hotplug when
> used and unplug when idle) and have at least interrupt lines wired to
> the SoC using a normal interrupt outside the enumerable bus.

That is indeed more difficult, because it requires geniune cooperation
between the bus and platform subsystems.

> I know there's been some discussion of this topic but do we have any
> general consensus on how to handle such things both from a Linux driver
> model point of view and from a DT/ACPI point of view?

The discussion involved some sort of pattern matching based on the
device name and the bus, but nothing was ever settled.

Alan Stern

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