linux-kernel - Re: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software Interface

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Message-ID: <1455792003.1384.33.camel@suse.com>
Date:	Thu, 18 Feb 2016 11:40:03 +0100
From:	Oliver Neukum <oneukum@...e.com>
To:	Felipe Balbi <balbif@...il.com>
Cc:	Felipe Balbi <balbi@...nel.org>,
	Heikki Krogerus <heikki.krogerus@...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: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software
 Interface

On Thu, 2016-02-18 at 12:30 +0200, Felipe Balbi wrote:
> Hi,
> 
> Oliver Neukum <oneukum@...e.com> writes:
> >> > What exactly are you sure about about?
> >> 
> >> heh, missed a NOT there :-)
> >
> > I am still confused :-)
> > Do you think a sysfs interface is good, bad or good
> > but insufficient?
> 
> I'm not sure it's the best interface. My fear is that as new
> requirements/features come along, the amount of files will continue to
> grow.

That will happen. The alternative, however is a "typectool" or
"usbpdtool" which would need to be updated for new features.

> >> I guess what I'm trying to say is that CC microcontroller might not be
> >> the one controlling the multiplexer which switches USB pins to another
> >> function. IOW:
> >> 
> >> Change to alternate mode X message
> >>  CC microcontroller interrupts CPU
> >>   read status to get X
> >>    change multiplexer
> >> 
> >
> > Yes. But it seems to me that in this case we need a kernel driver
> > without an API to user space. There are necessarily internal users
> 
> that assumes kernel always knows all possible alternate modes. What do
> we about bogus requests (request alternate mode X when X is not
> supported) ?

Do as the spec says: NACK it.

The questions which modes we offer, if we are a slave, still remains.
And I think the API is deficient in that regard. But again that question
is orthogonal of both issue, handling of bogus requests and how the
CC pins are exported.

	Regards
		Oliver