Subject: Re: [PATCH v17 2/3] usb: USB Type-C connector class
To: Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Mats Karrman <mats.dev.list@gmail.com>
References: <20170221142405.76299-1-heikki.krogerus@linux.intel.com>
 <20170221142405.76299-3-heikki.krogerus@linux.intel.com>
 <ae5d8f3a-859b-c3fa-1427-c25920a82abe@gmail.com>
 <4b4bbffc-db02-3b54-04bc-e7de79b2d9ed@roeck-us.net>
 <07618170-d561-e7fe-08e0-91316c53d832@gmail.com>
 <20170303125940.GA6999@kuha.fi.intel.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
        Felipe Balbi <felipe.balbi@linux.intel.com>,
        Oliver Neukum <oneukum@suse.com>, linux-kernel@vger.kernel.org,
        linux-usb@vger.kernel.org
From: Guenter Roeck <linux@roeck-us.net>
Message-ID: <e60de9cf-e454-69da-c2c6-6e33bc618f91@roeck-us.net>
Date: Fri, 3 Mar 2017 06:49:54 -0800
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On 03/03/2017 04:59 AM, Heikki Krogerus wrote:
> Hi,
>
> On Fri, Mar 03, 2017 at 08:29:18AM +0100, Mats Karrman wrote:
>> On 2017-03-03 04:13, Guenter Roeck wrote:
>>
>>> On 03/02/2017 07:22 AM, Mats Karrman wrote:
>>>> ....
>>>> Looking forward, one thing I have run into is how to connect the typec driver with a
>>>> driver for an alternate mode. E.g. the DisplayPort Alternate Mode specification
>>>> includes the HPD (hot plug) and HPD-INT (hot plug interrupt) signals as bits in the
>>>> Attention message. These signals are needed by the DisplayPort driver to know when to
>>>> start negotiation etc.
>>>> Have you got any thoughts on how to standardize such interfaces?
>
> My idea was to have something like the altmode "bus" at one point.
> We create a device for every alternate mode registered in typec class,
> so the alternate modes registered for the ports and partners would
> simply be attached to the altmode bus. There would be a bus per port
> of course.
>
> The drivers for the port alternate modes would take care of things
> like muxing and other platform specific stuff as needed, and they
> would be tied to the underlying subsystems and drivers, graphics in
> case of DisplayPort. The drivers for the partner alternate modes would
> take care of the actual communication with the alternate mode with
> VDMs if needed (but not necessarily), and they would need to be tied
> to the port alternate modes. In practice the driver for both the port
> and the partner alternate modes will be the same (in the same
> location) obviously, at least in most cases.
>
> I think a bus would allow us to support several ways of handling the
> alternate modes on different platforms. It would work fine also on
> platforms that had no use for it of course, like platforms where
> firmware or EC takes care of most things related to Type-C.
>
> But please note that since this is just a high level idea still, we
> wouldn't for example need to create an actual bus if there is no use
> for it, but since we have the SVIDs that can be used for matching,
> then why not try take advantage of them, right.
>
> How would something like that sound to you guys?
>
>>> That really depends on the lower level driver. For Chromebooks, where the Type-C
>>> Protocol Manager runs on the EC, we have an extcon driver which reports the pin states
>>> to the graphics drivers and connects to the Type-C class code using the Type-C class
>>> API. I still need to update, re-test, and publish that code. The published code in
>>> https://chromium.googlesource.com/chromiumos/third_party/kernel/, branch chromeos-4.4,
>>> shows how it can be done, though that code currently still uses the Android Type-C
>>> infrastructure.
>
> In this case I think you would only need to register a driver with the
> bus in case you want the handle to the device for the alternate mode.
>
>> OK, thanks!
>>
>> My system is a bit different. It's an i.MX6 SoC with the typec phy and DP controller connected
>> directly to the SoC and it's using DTB/OF.
>
> Is this "DP controller" a controller that is capable of taking care of
> the USB Power Delivery communication with the partner regarding
> DisplayPort alternate mode?
>
>> Using extcon I would have a driver that is both typec class and extcon driver at the same time
>> since I can't share the access to the typec phy. Is this done elsewhere in the kernel?
>> I don't know much about the wcove PMIC and what alternate modes it might support but I
>> guess that driver would end up in the same place.
>
> What alternate modes systems with WhiskeyCove supports depends on
> the platform. WhiskeyCove PMIC (as in Power Management IC) is
> available on a few Intel Atom platforms. The USB Type-C PHY in it
> provides a simple USB PD transceiver that does not touch the actual
> communication with the partners. The communication needs to be done in
> software, including dealing with alternate modes.
>
> I'm not planning on using extcon for anything with WhiskeyCove. I
> don't have any use for it. It looks to me that extcon is used just as
> a tool to create software couplings in many cases, and I'm not
> completely comfortable with that.
>

For rk3399 we went back and forth trying other approaches, but using extcon
turned out to be the most straightforward mechanism; everything else would
just have replicated what extcon already provides. Having said that,
I am open to other approaches. The rk3399 implementation may be seen as an
example on what is required, specifically when it comes to the interaction
between cable, DP, and the Type-C phy.

Guenter

> In my case with DP altmode, if we had for example muxes to take care
> of, I don't think it would be a problem to tie the driver for the mux
> to the graphics directly, so basically make it part of the graphics
> stack. That driver would be the port (and partner) altmode driver.
>
>> Do we need to further standardize attributes under (each) specific alternate mode to
>> include things such as HPD for the DP mode?
>
> I'm not completely sure what kind of system you have, but I would
> imagine that if we had the bus, your DP controller driver would be the
> port (and partner) alternate mode driver. The bus would bind you to
> the typec phy.
>
>
> Thanks,
>