Re: [PATCH] arm: dtsi: am335x-bone-common, usb0 is peripheral only

[Tony Lindgren <tony@atomide.com>]

* Markus Pargmann <mpa@pengutronix.de> [140214 02:05]:
> Hi,
> 
> On Thu, Feb 13, 2014 at 03:25:52PM -0800, Tony Lindgren wrote:
> > * Markus Pargmann <mpa@pengutronix.de> [140213 15:16]:
> > > Hi,
> > > 
> > > On Thu, Feb 13, 2014 at 02:54:38PM -0800, Tony Lindgren wrote:
> > > > * Markus Pargmann <mpa@pengutronix.de> [140111 06:03]:
> > > > > The PMIC is using usb0 vbus line as power source. It is also connected
> > > > > to the am335x processor as vbus sense. But there is no possibility to
> > > > > pullup usb0 vbus to operate as host. This patch fixes the dr_mode of usb0.
> > > > 
> > > > That's the MUSB? AFAIK it's not possible to operate MUSB in peripheral
> > > > only mode because the hardware does what it wants based on the ID
> > > > pin state.
> > > 
> > > Yes that's MUSB. The am335x reference manual describes that it is
> > > possible to force peripheral/host mode by setting bit 7 (IDDIG_MUX) in
> > > register USBnMODE to 1. Then it uses the bit written in bit 8 (IDDIG) of
> > > register USBnMODE to set host/peripheral mode.
> > 
> > OK
> >  
> > > I am not sure if the driver supports it yet but I think the DTS should
> > > contain the correct mode nevertheless, especially to avoid starting the
> > > otg loops in the musb driver.
> > 
> > Well there's one more thing to consider.. I think in the OTG role change
> > case the VBUS is still driven externally from the original host, so the
> > lack of VBUS does not always mean that host mode should be disabled.
> 
> I thought more about the hardware description than the possible role
> changes through software protocols. In a hardware perspective, this USB
> port is only in peripheral mode, as it can't drive VBUS. However is
> there any support for role change protocols in the kernel yet?

There's some to trigger it, if you grep -ri hnp drivers/usb/
you'll see some references to it.
 
> Perhaps we have to add a seperate DT binding for usb role changes when
> they are supported. This would help to describe the hardware
> capabilities (host, peripheral or OTG) and the role change protocols
> supported.

Yeah it might be worth discussing on the USB list what's the best way
to deal with all that.

Regards,

Tony