linux-kernel - Using TCPM for ports without Power Delivery support

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Message-ID: <6cf411e1-8234-d1ca-541d-067665620945@microchip.com>
Date:   Mon, 15 Feb 2021 17:25:29 +0000
From:   <Cristian.Birsan@...rochip.com>
To:     <linux@...ck-us.net>, <heikki.krogerus@...ux.intel.com>,
        <gregkh@...uxfoundation.org>
CC:     <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Using TCPM for ports without Power Delivery support

Hi,

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?

Kind regards,
Cristian