linux-kernel - Re: [RFC/PATCH 1/2] usb: dwc3: disable for USB_ROLE

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Message-ID: <20250712001131.ax3rw24h3deekxfv@synopsys.com>
Date: Sat, 12 Jul 2025 00:11:38 +0000
From: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To: John Keeping <jkeeping@...usicbrands.com>
CC: "linux-rockchip@...ts.infradead.org" <linux-rockchip@...ts.infradead.org>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        Nicolas Frattaroli <nicolas.frattaroli@...labora.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-phy@...ts.infradead.org" <linux-phy@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/PATCH 1/2] usb: dwc3: disable for USB_ROLE_NONE

On Thu, Jul 10, 2025, John Keeping wrote:
> When the phy is acting as a Type C mux, it may need to reset when the
> cable direction changes.  But this should not happen while DWC3 is
> trying to use the USB connection.
> 
> In this case, there must be a connection manager to notify the phy of
> the orientation change and tcpm_mux_set() ensures this happens before
> DWC3's role switch is informed of a change.
> 
> It should not be possible to go directly from device->device or
> host->host with a change in orientation without transitioning through
> the "none" role as the cable is unplugged.  So ensuring that DWC3 always

The controller is either host or device mode. It's odd to use "none" to
indicate connection.

> informs the phy of the new mode whenever a plug is detected should be
> sufficient for the phy to safely reset itself at a time that is safe for
> DWC3.

Couldn't the phy do this as it detects connection/disconnection.

It seems what you need is a full controller initialization upon new
connection on orientation change, and you're using role_switch selecting
"none" to do so.

I'm not sure if role-switch has the right interface to do so. Perhaps we
can introduce one? I don't think we should change the behavior of the
current flow and apply that to all other platforms.

Thanks,
Thinh

> 
> Lifting the special-case for desired_dr_mode==0 in __dwc3_set_mode() is
> sufficient to allow using the unset mode for USB_ROLE_NONE.  The
> handling already disables the old mode and then simply does not enable a
> new one.
> 
> If an external device is notifying USB role switches, then it is not
> necessary to set the default role when USB_ROLE_NONE is passed.