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

From: Oliver Neukum
Date: Tue May 24 2016 - 06:22:18 EST

On Tue, 2016-05-24 at 13:08 +0300, Heikki Krogerus wrote:
> Hi Guenter,
> On Mon, May 23, 2016 at 09:52:12AM -0700, Guenter Roeck wrote:
> > On Mon, May 23, 2016 at 05:55:04PM +0200, Oliver Neukum wrote:
> > > On Mon, 2016-05-23 at 07:43 -0700, Guenter Roeck wrote:
> > > > On 05/23/2016 06:58 AM, Oliver Neukum wrote:
> > >
> > > > > Now I am confused. Are you saying that the choice of Alternate Mode does
> > > > > not belong into user space?
> > > > >
> > > >
> > > > No; sorry for the confusion. The above was meant to apply to my use
> > > > of "preferred mode", not yours. I was trying to say that the choice of
> > > > preferred roles (which determines if Try.SRC or Try.SNK is enabled)
> > > > should belong primarily into the kernel, to be determined by the platform
> > > > (presumably via ACPI, devicetree data, or platform data). If it should
> > >
> > > Why on earth? That is most clearly a policy decision.
> > >
> >
> > The question is not that much if it is policy (it is), but if the policy
> > should be driven by the platform or by user space. I think there needs
> > to be at least a default driven by the platform. As already mentioned,
> > I am ok with a means to override this platform default from user space.
> > But if user space doesn't say, there still needs to be a default.
> I don't completely agree with that. The platform should not, and
> actually in most cases with ACPI AFAIK, will not provide any
> "preferences" to the OS about anything. The platform should only
> provide the OS the physical capabilities and nothing else. So if for
> example the platform is capable of acting as only source with a Type-C
> port, that is what it needs to tell to the OS so possibly the PHY can
> be programmed accordingly, etc.

May I suggest that the point is moot as long as we agree that user space
needs to be able to set a policy? The OS cannot ignore the port before
user space tells it what to do. So a default will be needed.

The OS must act conservatively, which means it cannot deactivate
hardware. So if a port can do DRP, that must be the default.
I would go so far that I would suggest that we add a module parameter
for Try.SRC and Try.SNK to avoid trouble during boot.

> So IMO, just like with any decision related to what the system will
> ultimately be used for, decision about the preferred role really
> belongs to the userspace.

Yes, but ought the APIs for role, mode and PD be separate or not.
Sorry to sound like a broken record, but you need to provide some
higher level planning here.