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

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Date:	Wed, 17 Feb 2016 15:36:46 +0200
From:	Felipe Balbi <balbi@...nel.org>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Oliver Neukum <oneukum@...e.com>
Cc:	Felipe Balbi <balbif@...il.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


Hi,

Heikki Krogerus <heikki.krogerus@...ux.intel.com> writes:
> On Wed, Feb 17, 2016 at 11:36:52AM +0100, Oliver Neukum wrote:
>> On Wed, 2016-02-17 at 12:29 +0200, Felipe Balbi wrote:
>> > Hi,
>> > 
>> > Oliver Neukum <oneukum@...e.com> writes:
>> > > On Wed, 2016-02-17 at 09:58 +0200, Heikki Krogerus wrote:
>> > >> On Tue, Feb 16, 2016 at 02:39:47PM +0100, Oliver Neukum wrote:
>> 
>> > >> > Yes, but we need an API. We can't keep adding to it. So if that
>> > >> > is to be supported, it needs to be defined now.
>> > >> 
>> > >> When you say API, do you mean the API the class provides to the
>> > >> drivers? Or did you mean ABI which would be the sysfs in this case?
>> > >
>> > > The API to user space. That is the point. We cannot break user space.
>> > > Once this sysfs API is upstream we are stuck with it.
>> > 
>> > yeah, in fact I have been wondering if sysfs is the best interface to
>> 
>> That is the discussion we must have.
>> 
>> > userspace. I talked with Heikki a few days back about this; I was
>> > wondering if something like what the NFC folks did with netlink would be
>> > better here.
>> 
>> I doubt that, because the main user is likely to be udev scripts.
>> They can easily deal with sysfs attributes.
>
> IMHO for high level interface like this, sysfs is ideal because of the
> simple fact that you only need a shell to access the files. netlink
> would make us depend on custom software, no?
>
> I'm not against using netlink, but what would be the benefit from it
> in this case?

With HW we see nowadays, CC stack is hidden on some microcontroller, but
is it too far-fetched to consider a system where this is not the case ?

Specially when we consider things like power delivery which, I know, you
wanted to keep it out of this interface, however we would have two
'stacks' competing for access to the same pins, right ?

IIRC mode and role negotiation goes via CC pins using the power delivery
protocol. If I misunderstand anything, let me know.

-- 
balbi

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