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

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Date:   Tue, 30 Aug 2016 13:16:46 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Vincent Palatin <vpalatin@...omium.org>,
        Bin Gao <bin.gao@...ux.intel.com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCHv6 1/3] usb: USB Type-C connector class

On Tue, 2016-08-30 at 13:04 +0300, Heikki Krogerus wrote:
> On Tue, Aug 30, 2016 at 11:32:01AM +0200, Oliver Neukum wrote:

Hi,

> > 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?

No, it does not.

> > 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

Error reporting does not require a synchronous operation. Reporting
it in the next read() or write() and making it pollable is perfectly
viable. It just must not be silently dropped.

	Regards
		Oliver