linux-kernel - Re: [PATCHv6 1/3] usb: USB Type-C connector class

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Date:   Tue, 30 Aug 2016 17:13:31 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Oliver Neukum <oneukum@...e.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Vincent Palatin <vpalatin@...omium.org>,
        Bin Gao <bin.gao@...ux.intel.com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCHv6 1/3] usb: USB Type-C connector class

On Tue, Aug 30, 2016 at 06:46:24AM -0700, Guenter Roeck wrote:
> On 08/30/2016 03:04 AM, Heikki Krogerus wrote:
> > Hi Oliver,
> > 
> > On Tue, Aug 30, 2016 at 11:32:01AM +0200, Oliver Neukum wrote:
> > > On Mon, 2016-08-29 at 15:36 +0300, Heikki Krogerus wrote:
> > > > +What:          /sys/class/typec/<port>/current_data_role
> > > > +Date:          June 2016
> > > > +Contact:       Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > > > +Description:
> > > > +               The current USB data role the port is operating in.
> > > > This
> > > > +               attribute can be used for requesting data role
> > > > swapping on the
> > > > +               port. Swapping is only supported as an asynchronous
> > > > operation
> > > > +               and requires polling of the attribute in order to know
> > > > the
> > > > +               result, so successful write operation does not mean
> > > > successful
> > > > +               swap.
> > > > +
> > > 
> > > That is badly formulated. Does it mean that poll() or select()
> > > can be used or does the value need to be repearedly read?
> > 
> > Does polling not always mean poll/select?
> > 
> > > And how would you learn about an error?
> > 
> > This is what I'm also really worried about. I'm now wondering did I
> > give up too easily on this to Guenter in hope to move this thing
> > forward. He said it's problematic to do these calls synchronously for
> > him. Was it something related to potential conflicting role swaps from
> > both ends?
> > 
> > Guenter, can you please elaborate? And how do you plan to report
> > failures with the swaps?
> > 
> 
> I thought we had this sorted out. When I said "asynchronous", I did not mean
> that the sysfs operation would not wait for the operation to complete. I meant
> that the Type-C state machine operates in a different context than the sysfs/class
> code. Since the state machine operates in a different context, it may have
> to execute a callback into the class code at any time, independently of
> any pending role changes triggered through sysfs. Please have a look into
> the patch set I submitted for details. Roughly it works as follows.
> 
> Class code context				State machine context
> 
> User requests role change
> Class code calls {dr,pr,vconn}_set
> {dr,pr,vconn}_set code validates request
> {dr,pr,vconn}_set code sends role change
> 	request to state machine		State machine gets role change request
> {dr,pr,vconn}_set code waits for completion
> 						State machine sends role change request
> 						to link partner
> 						Partner reports Accept or Reject
> 						State machine changes state as requested
> 						State machine reports new role to class code
> 							via callbacks
> 						State machine informs Class code that request
> 						is complete
> {dr,pr,vconn}_set code gets results
> 	and returns to caller
> Class code reports results to user
> 
> From user perspective, everything is synchronous. However, the state machine has to be
> able to run independently and report role and other state changes to the class code while
> a role change request from the class code is pending. For example, it has to be able to
> handle incoming role change requests from the link partner, and it has to be able to
> handle link state changes. All those have to be reported to the class code. This is
> impossible if the class code holds a lock while a role change triggered from user space
> is pending, which is why I asked for the locks in the class code to be removed.
> 
> Maybe my use of the term "asynchronous" was misleading, and I should have said "operates
> in a different context" instead. My apologies.

Thanks for the explanation. I remember you did explain this before I
started my parental leave, but I forgot all about it.


Thanks,

-- 
heikki