linux-kernel - Re: [PATCH v10 2/7] usb: mux: add generic code for dual role port mux

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Date:	Fri, 3 Jun 2016 11:16:32 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Peter Chen <hzpeterchen@...il.com>
Cc:	Lu Baolu <baolu.lu@...ux.intel.com>, felipe.balbi@...ux.intel.com,
	Mathias Nyman <mathias.nyman@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Lee Jones <lee.jones@...aro.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, Roger Quadros <rogerq@...com>
Subject: Re: [PATCH v10 2/7] usb: mux: add generic code for dual role port mux

On Fri, Jun 03, 2016 at 03:41:13PM +0800, Peter Chen wrote:
> On Thu, Jun 02, 2016 at 09:37:24AM +0800, Lu Baolu wrote:
> > Several Intel platforms implement USB dual role by having completely
> > separate xHCI and dwc3 IPs in PCH or SOC silicons. These two IPs share
> > a single USB port. There is another external port mux which controls
> > where the data lines should go. While the USB controllers are part of
> > the silicon, the port mux design are platform specific.
> > 
> > This patch adds the generic code to handle such multiple roles of a
> > usb port. It exports the necessary interfaces for other components to
> > register or unregister a usb mux device, and to control its role.
> > It registers the mux device with sysfs as well, so that users are able
> > to control the port role from user space.
> > 
> > Some other archs (e.g. Renesas R-Car gen2 SoCs) need an external mux to
> > swap usb roles as well. This code could also be leveraged for those archs.
> > 
> 
> Sorry to review this so late, from my point, it is a dual-role switch
> driver too, we are reviewing USB OTG/dual-role framework [1], it is
> not necessary to create another framework to do it. And USB OTG framework
> has already tested at Renesas's platform [2].
> 
> [1] http://www.spinics.net/lists/linux-usb/msg140835.html
> [2] http://www.spinics.net/lists/linux-usb/msg140827.html

We really can't marry dual-role capability with OTG. That OTG
framework can be used be when the hardware actually supports the
protocols defined in the OTG spec starting from SRP. In other cases it
must not be used.

OTG relies heavily on existence of the ID pin, but with Type-C
connectors we do not have it. Therefore USB Type-C defines competing
support for example for the role swapping. With USB Type-C connectors
OTG will never be supported.

So let's not mix USB dual-role capability with OTG.


Thanks,

-- 
heikki