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Date:   Tue, 16 Feb 2021 11:12:25 +0200
From:   Heikki Krogerus <heikki.krogerus@linux.intel.com>
To:     Cristian.Birsan@microchip.com
Cc:     linux@roeck-us.net, gregkh@linuxfoundation.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Using TCPM for ports without Power Delivery support
Message-ID: <YCuMeW6lbaSq8CIM@kuha.fi.intel.com>
References: <6cf411e1-8234-d1ca-541d-067665620945@microchip.com>
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Hi Cristian,

On Mon, Feb 15, 2021 at 05:25:29PM +0000, Cristian.Birsan@microchip.com wrote:
> My name is Cristian and I'm working on bringing up a USB Type-C Port Controller
> (TCPC) without Power Delivery support which is intended to work with USB 2.0
> Host/Device.
> 
> The IP is integrated into one of Microchip's SoCs, it is memory-mapped and it
> was designed based on USB Type-C Cable and Connector specification revision 1.2.
> 
> In brief, it has support for detecting the threshold voltages on CC1, CC2 lines,
> control of the current source (Ip), and pull-down resistors (Rd). The management
> of the controller is to be implemented in software (it is not autonomous).
> 
> Having in mind that the controller uses proprietary registers, I chose to
> implement it using TCPM directly and skip the TCPC Interface.
> 
> For the beginning, I would like to enable simple use cases like the ones
> described in Connection State Diagram: Source and Connection State Diagram: Sink
> from USB Type-C Cable and Connector Specification.
> 
> Some of the problems that I encountered until now are:
> 
> 1. tcpm_register_port() fails if set_pd_rx(), pd_transmit() or set_vconn()
> functions are missing.
> 
> 2. the port capabilities are specified in the connector DT bindings only through
> PDOs, even though PDOs are specific to PD mode.
> 
> 3. once I was able to start the TCPM state machine, it called pd_transmit() in
> the process to negotiate the capabilities. For my case I used a dummy function
> just to be able to register the port.
> 
> Please let me know what you think and if you have any advice. Am I going in the
> right direction or is there a better way to implement this?

Don't bother with tcpm if you don't have PD support. Just register
your port(s) and the partners directly with the connector class:
https://www.kernel.org/doc/html/latest/driver-api/usb/typec.html

You can use the driver for the TI HD3SS3220 controller as an example
how to do that (drivers/usb/typec/hd3ss3220.c). That thing is also
just USB Type-C PHY without PD support just like your port controller.


Br,

-- 
heikki