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

[Oliver Neukum]

On Tue, 2016-06-21 at 16:58 +0300, Heikki Krogerus wrote:
> On Tue, Jun 21, 2016 at 03:08:52PM +0200, Oliver Neukum wrote:
>  
> > 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?
> 
> Systems where the firmware has to negotiate PD will likely provide
> firmware interface like UCSI, and where the OS has no direct
> interaction with the USB PD transceiver. In these case there is no
> need to renegotiate as we are just reporting in OS the initial state
> after bootup.

How certain is that? I was under the impression that on many systems
the OS would speak to the TCPM directly.

> We do have a system where the typec port is used to power the board.
> On these systems the firmware does not communicate PD (so we will
> never have the firmware displaying anything over Type-C on those
> systems), but the USB PD chargers for example are detected as 3.0A
> Type-C power supplies before any USB PD negotiation takes place, just
> like the spec says, and that is more then enough to power these boards.

Now correct me, if I am misreading the spec. I am sure the system
will boot unless it needs ridiculous amounts of power, but
will we see anything on the screen? As far as I can tell the spec
actually says that you cannot enter an alternate mode without having
established a power contract.
If we really leave entering modes up to user space, we have lost
printk before getting into the initrd at the earliest.

	Regards
		Oliver