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

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Message-ID: <20160531124355.GC10084@kuha.fi.intel.com>
Date:	Tue, 31 May 2016 15:43:56 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Oliver Neukum <oneukum@...e.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 Tue, May 31, 2016 at 03:09:01PM +0300, Heikki Krogerus wrote:
> On Tue, May 31, 2016 at 10:48:29AM +0200, Oliver Neukum wrote:
> > On Tue, 2016-05-31 at 11:31 +0300, Heikki Krogerus wrote:
> > > Hi Oliver,
> > > 
> > > On Mon, May 30, 2016 at 03:59:27PM +0200, Oliver Neukum wrote:
> > > > On Mon, 2016-05-30 at 16:19 +0300, Heikki Krogerus wrote:
> > > > > Hi guys,
> > > > > 
> > > > > I'm attaching a diff instead of full v3. I'm not yet adding attributes
> > > > > for the reset and cable_reset. I still don't understand what is the
> > > > > case where the userspace would need to be able to tricker reset? Why
> > > > > isn't it enough for the userspace to be able to enter/exit modes?
> > > > > Oliver! Can you please comment?
> > > > 
> > > > 1. Because we need error handling.
> > > >    Devices crash. Cables will crash. We will get out of sync.
> > > >    You never put yourself in a place where you cannot handle an
> > > >    IO error.
> > > > 2. Because it is in the spec. We do not second guess the spec.
> > > >    We implement it.
> > > 
> > > Error conditions and crashes are the responsibility of the USB PD
> > > stack, not userspace. In those cases the stack can not wait for a
> > 
> > Those are not exclusive conditions.
> > 
> > > command from the userspace. So for example if a timer like
> > > NoResponseTimer times out, the stack an its state machines will have
> > > to take care of the reset quite independently.
> > 
> > Yes. But somebody needs to handle high level errors.
> > 
> > > If you get out of sync with an alternate mode, you reset that specific
> > > alternate mode by exiting and re-entering it, and you do not reset the
> > > entire PD connection, port, partner or cable.
> > 
> > That would be the first step. If that doesn't work you will at that
> > point either give up or use the next largest hammer.
> > In principle you could do that in kernel space, but that implies
> > that the kernel can detect all failures. That is unlikely.
> 
> Any PD communication failures the kernel has to be able to detect, so
> I guess you mean failures with the alternate modes themselves, right?
> 
> In that case, surely exiting the mode is enough to "reset" it? When it
> is re-entered, it has to be completely re-configured in any case. I
> don't see how resetting the whole port or cable would guarantee that a
> mode would become any more functional in case of failures? It will
> however make also the other active modes to de-activate even if they
> are functioning fine.

Forget about it, I'll just add the reset attributes. I'm still not
clear about their usefulness, but instead they will just create a small
risk, but I can live with that.


Cheers,

-- 
heikki