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

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Message-ID: <20160603092039.GA24714@shlinux2>
Date:	Fri, 3 Jun 2016 17:20:40 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.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 11:16:32AM +0300, Heikki Krogerus wrote:
> 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.
> 

Well, DRD/OTG framework is mainly used for dual-role switch, no
matter what input is, you can use id pin, sysfs, or Type-C events.
It is long term target, currently, it only supports id pin.
In future, we can expend it to support more input events.
Microsoft also has similar framework for it:

https://msdn.microsoft.com/en-us/library/windows/hardware/dn957036(v=vs.85).aspx

-- 

Best Regards,
Peter Chen