linux-kernel - Re: [PATCH v17 2/3] usb: USB Type-C connector class

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Date:   Mon, 6 Mar 2017 15:16:19 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Peter Chen <hzpeterchen@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Oliver Neukum <oneukum@...e.com>, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, Roger Quadros <rogerq@...com>,
        Jun Li <jun.li@....com>
Subject: Re: [PATCH v17 2/3] usb: USB Type-C connector class

Hi Peter,

On Mon, Mar 06, 2017 at 09:15:51AM +0800, Peter Chen wrote:
> > > What interface you use when you receive this event to handle
> > > dual-role switch? I am wonder if a common dual-role class is
> > > needed, then we can have a common user utility.
> > > 
> > > Eg, if "data_role" has changed, the udev can echo "data_role" to
> > > /sys/class/usb-dual-role/role
> > 
> > No. If the partner executes successfully for example DR_Swap message,
> > the kernel has to take care everything that is needed for the role to
> > be what ever was negotiated on its own. User space can't be involved
> > with that.
> > 
> 
> Would you give me an example how kernel handle this? How type-C event
> triggers role switch?

On our boards, the firmware or EC (or ACPI) configures the hardware as
needed and also notifies the components using ACPI if needed. It's
often not even possible to directly configure the components/hardware
for a particular role.

I'm not commenting on Roger's dual role patch series, but I don't
really think it should be mixed with Type-C. USB Type-C and USB Power
Delivery define their own ways of handling the roles, and they are not
limited to the data role only. Things like OTG for example will, and
actually can not be supported. With Type-C we will have competing
state machines compared to OTG. The dual-role framework may be useful
on systems that provide more traditional connectors, which possibly
have the ID-pin like micro-AB, and possibly also support OTG. It can
also be something that exist in parallel with the Type-C class, but
there just can not be any dependencies between the two.

Thanks,

-- 
heikki