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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
