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

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Date:	Wed, 22 Jun 2016 12:08:57 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	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>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [RFC PATCHv2] usb: USB Type-C Connector Class

On Wed, 2016-06-22 at 12:31 +0300, Heikki Krogerus wrote:

Hi,

> > 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.
> 
> With these boards, you will not see anything on the screen that is
> attached to a Type-C connector until the OS has booted to the point
> where it has negotiated the power contract and entered a mode.
> 
> If the system has BIOS/FW/EC capable of negotiating the power contract
> and enter a mode, but where we still are expected to take over the
> whole TCPM in OS, I think the connection will be reset.

Think about a DP over type C display with a USB PD power brick on a
daisy chain.
If the host needs more than 15W or more than 5V, a reset is suicide.

And losing earlyprintk hurts a lot.
This means we need USB PD statically in the kernel. And a kernel
based policy that brings up all displays.

	Regards
		Oliver