linux-kernel - Re: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software Interface

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Message-ID: <CAOiXhaKYf0XGTNYF=dmwoSZKNGBY+niXHXK9Veo8tSsUpcBeyA@mail.gmail.com>
Date:	Thu, 18 Feb 2016 16:07:54 +0530
From:	Rajaram R <rajaram.officemail@...il.com>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:	Felipe Balbi <balbi@...nel.org>, Oliver Neukum <oneukum@...e.com>,
	Felipe Balbi <balbif@...il.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software Interface

On Wed, Feb 17, 2016 at 7:58 PM, Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
> On Wed, Feb 17, 2016 at 03:36:46PM +0200, Felipe Balbi wrote:
>>
>> Hi,
>>
>> Heikki Krogerus <heikki.krogerus@...ux.intel.com> writes:
>> > On Wed, Feb 17, 2016 at 11:36:52AM +0100, Oliver Neukum wrote:
>> >> On Wed, 2016-02-17 at 12:29 +0200, Felipe Balbi wrote:
>> >> > Hi,
>> >> >
>> >> > Oliver Neukum <oneukum@...e.com> writes:
>> >> > > On Wed, 2016-02-17 at 09:58 +0200, Heikki Krogerus wrote:
>> >> > >> On Tue, Feb 16, 2016 at 02:39:47PM +0100, Oliver Neukum wrote:
>> >>
>> >> > >> > Yes, but we need an API. We can't keep adding to it. So if that
>> >> > >> > is to be supported, it needs to be defined now.
>> >> > >>
>> >> > >> When you say API, do you mean the API the class provides to the
>> >> > >> drivers? Or did you mean ABI which would be the sysfs in this case?
>> >> > >
>> >> > > The API to user space. That is the point. We cannot break user space.
>> >> > > Once this sysfs API is upstream we are stuck with it.
>> >> >
>> >> > yeah, in fact I have been wondering if sysfs is the best interface to
>> >>
>> >> That is the discussion we must have.
>> >>
>> >> > userspace. I talked with Heikki a few days back about this; I was
>> >> > wondering if something like what the NFC folks did with netlink would be
>> >> > better here.
>> >>
>> >> I doubt that, because the main user is likely to be udev scripts.
>> >> They can easily deal with sysfs attributes.
>> >
>> > IMHO for high level interface like this, sysfs is ideal because of the
>> > simple fact that you only need a shell to access the files. netlink
>> > would make us depend on custom software, no?
>> >
>> > I'm not against using netlink, but what would be the benefit from it
>> > in this case?
>>
>> With HW we see nowadays, CC stack is hidden on some microcontroller, but
>> is it too far-fetched to consider a system where this is not the case ?
>
> There already are several USB PD stacks out there, like also Greg
> pointed out.
>
>> Specially when we consider things like power delivery which, I know, you
>> wanted to keep it out of this interface, however we would have two
>> 'stacks' competing for access to the same pins, right ?
>
> No. This class would be the top layer for the coming stack, where ever
> it ends up coming. The class is only the interface to the user space
> and nothing else.
>
> By saying we need to keep USB Type-C separate from USB PD I meant that
> the userspace access can not be mixed somewhere in layers of the USB
> PD/CC stack like it has been in the USB PD stacks I've seen so far.
> They assume that we always use the software USB PD stack with USB
> Type-C, which as we can see is not true when the stack is implemented
> in EC or firmware or some complex USB PD controller or what ever.
> However, the operations the userspace needs to do are exactly the same
> in both cases.
>
> - data role swapping
> - power role swapping (depends on USB PD)
> - Alternate Modes (depends on USB PD)
>
> And we really should not forget that we actually also have USB Type-C
> PHYs that can't do any USB PD communication over the CC pin, so USB PD
> is simply not always going to be available. But the data role swapping
> and also accessories are still available with them, as the do not need
> USB PD.
>
> This was the whole point with the class. It allows the different ways
> of dealing with Type-C ports to be exposed to userspace in the same
> way.
>
>> IIRC mode and role negotiation goes via CC pins using the power delivery
>> protocol. If I misunderstand anything, let me know.
>
> The data role swap with USB Type-C connectors is in no way tied to USB
> Power Delivery. The USB Type-C spec defines that when USB PD is

Its not data role swap i guess its dual role, A Data role swap is tied
with USB PD,

> available, DR_Swap USB PD function is used to swap the role, otherwise
> emulated disconnect will do the trick.

I doubt a USB host with no device capability implement DRP ?? Also
emulated trick(??) is not spec requirement rt ?

>
> Data role swapping is a must thing to have with USB Type-C connectors

I guess you are referring to Dual role (DRP) and not data role (DRD).

> because of the fact that the role is selected randomly. Regardless was
> USB PD supported or not.
>
>
> Thanks,
>
> --
> heikki
> --
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