linux-kernel - Re: [PATCH v17 2/3] usb: USB Type-C connector class

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Date:   Wed, 8 Mar 2017 06:44:47 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Peter Chen <peter.chen@....com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     Peter Chen <hzpeterchen@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Oliver Neukum <oneukum@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Roger Quadros <rogerq@...com>, Jun Li <jun.li@....com>
Subject: Re: [PATCH v17 2/3] usb: USB Type-C connector class

On 03/07/2017 10:50 PM, Peter Chen wrote:
>
>>>> You mean type-C trigger an ACPI event, and this ACPI event can notify
>>>> related USB controller driver doing role switch?
>>>
>>> No (firmware programs the dual-role hw/registers), but never mind.
>>> That could be the case.
>>>
>>>> If it is correct, there is a notifier between type-C and USB
>>>> controller driver, how to define this notifier for non-ACPI platform?
>>>
>>> Once there is a platform with Type-C like that, the problem needs to
>>> be solved. However..
>>>
>>>>> I'm not commenting on Roger's dual role patch series, but I don't
>>>>> really think it should be mixed with Type-C. USB Type-C and USB
>>>>> Power Delivery define their own ways of handling the roles, and they
>>>>> are not limited to the data role only. Things like OTG for example
>>>>> will, and actually can not be supported. With Type-C we will have
>>>>> competing state machines compared to OTG. The dual-role framework
>>>>> may be useful on systems that provide more traditional connectors,
>>>>> which possibly have the ID-pin like micro-AB, and possibly also
>>>>> support OTG. It can also be something that exist in parallel with the Type-C
>> class, but there just can not be any dependencies between the two.
>>>>>
>>>>
>>>> Yes, there are two independent things. But if the kernel doesn't have
>>>> a notifier between type-C message sender (type-c class) and message
>>>> receiver (like USB controller driver for role switch or other drivers
>>>> for alternate mode message), we had to find some ways at userspace.
>>>
>>> ..what ever the solution is, it really can't rely on user space.
>>>
>>
>> ... and, at least for our application, using extcon for the necessary notifications works
>> just fine.
>>
>
> I see, that means you have a hardware signal to notify role switch.
>

In our case the Type-C protocol manager (including alternate mode handling)
is implemented in an EC. The EC signals the extcon-cros_ec driver, which
in turn signals the phy driver as well as the DP driver. The Type-C class
is orthogonal; extcon-cros_ec will also register with the Type-C class code
once that is upstream.

As mentioned earlier, using extcon for signaling was the most convenient means
for us to pass events around. I am more than open to change it to a bus,
if that can be made to work - we'd have to keep in mind though that this code
already works without Type-C infrastructure and is for the most part already
upstream (the rk3399 code it ties into is upstream, and extcon-cros_ec has been
submitted as https://patchwork.kernel.org/patch/9583045/).

As for to how to handle alternate mode if the Type-C protocol manager
(TCPM) is implemented in the kernel - I have not yet implemented it yet,
but my thinking goes along the line described by Heikki in his last e-mail.

Note that we also have a kernel driver for FUSB302 which ties into my tcpm
driver. I'll have to check if that is public yet and if I or someone
else can publish it if there is interest.

Guenter