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

[Mark Rutland <mark.rutland@arm.com>]

[Adding Olof]

On Mon, Aug 12, 2013 at 10:51:36AM +0100, Mark Brown wrote:
> On Sun, Aug 11, 2013 at 09:53:01PM -0400, Alan Stern wrote:
> > On Sun, 11 Aug 2013, Mark Brown wrote:
> 
> > > 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?
> 
> I tried that, it doesn't seem to work - for some reason it seems that
> the hub is only successfully enumerated if it starts after its parent is
> running.  I don't know enough about USB to speculate on why that might
> be, the GPIOs are brining the device out of reset not applying power or
> anything.
> 
> > > 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.
> 
> Yeah.  You might want to do the same with for example a USB network
> controller when you're in flight only mode, that seems to be one of the
> more common reasons for doing this sort of thing with USB.

As I understand it, the wifi chip on the Snow Chromebook has a similar
issue -- it hangs off of a probeable SDIO bus, but needs a regulator
poked for it to turn on and become probeable (see
exynos_wifi_bt_set_power in [1]).

Thanks,
Mark.

[1] http://git.chromium.org/gitweb/?p=chromiumos/third_party/kernel.git;a=blob;f=arch/arm/mach-exynos/mach-exynos5-dt.c;h=d131241ea78b7d89df21e676e284ddfd369b4da0;hb=refs/heads/chromeos-3.4