linux-kernel - Re: [RFC PATCHv2] usb: USB Type-C Connector Class

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Date:	Tue, 21 Jun 2016 06:24:48 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Oliver Neukum <oneukum@...e.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:	Andy Shevchenko <andy.shevchenko@...il.com>,
	Rajaram R <rajaram.officemail@...il.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [RFC PATCHv2] usb: USB Type-C Connector Class

On 06/21/2016 06:08 AM, Oliver Neukum wrote:
> On Thu, 2016-05-19 at 15:44 +0300, Heikki Krogerus wrote:
>> The purpose of this class is to provide unified interface for user
>> space to get the status and basic information about USB Type-C
>> Connectors in the system, control data role swapping, and when USB PD
>> is available, also power role swapping and Alternate Modes.
>
> This raises two more questions.
>
> 1. Booting
>
> It is possible that our only display and, worse, our source
> of power is a display that can be used only in an alternate mode
> and is connected via a type C connector.
>
> We need some kind of boot time support for alternate modes.
>
> The firmware will surely want to display something. So it is possible
> that we start the OS will a valid power contract. How do we deal
> with that? Renegotiate?
>

In one of my drivers, the PD protocol is running on an EC and the Linux
driver is just interfacing it to the typec class. I don't do any renegotiating
but just report the port state to the class. What is wrong with that, and why
would it not work ?

If the PD protocol runs in Linux (which I implement as well), I tried renegotiating,
ie assuming that a contract was already established, with several multi-function
adapters. Quite often those 'die' when trying tricks like that, and have to be manually
disconnected from power (on both ends) to get back to life. Granted, part of that
is that the firmware on those adapters is early and not as stable as I would like it
to be, but still that is a significant risk. I ended up always starting with error
recovery to avoid that kind of problem, at least for now. Even that doesn't always
help.

 > 2. Multiple GPUs
>
> How do we know which GPU is connected to which port?
>

Still working on it, but the basic idea (in my case) is to use devicetree data
or platform data if that is not available.

Guenter