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

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Message-ID: <20160524100822.GB26874@kuha.fi.intel.com>
Date:	Tue, 24 May 2016 13:08:22 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Oliver Neukum <oneukum@...e.com>,
	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>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [RFC PATCHv2] usb: USB Type-C Connector Class

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.

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.


Thanks,

-- 
heikki