linux-kernel - RE: [PATCH] usb: dwc3: Allow usb role swich control from userspace

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Message-ID: <058201dc4403$334b0f20$99e12d60$@samsung.com>
Date: Thu, 23 Oct 2025 15:25:41 +0530
From: "Pritam Manohar Sutar" <pritam.sutar@...sung.com>
To: "'Thinh Nguyen'" <Thinh.Nguyen@...opsys.com>
Cc: <gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <rosa.pila@...sung.com>,
	<dev.tailor@...sung.com>, <faraz.ata@...sung.com>,
	<muhammed.ali@...sung.com>, <selvarasu.g@...sung.com>
Subject: RE: [PATCH] usb: dwc3: Allow usb role swich control from userspace

Hi Thinh,

> -----Original Message-----
> From: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
> Sent: 22 October 2025 03:40 AM
> To: Pritam Manohar Sutar <pritam.sutar@...sung.com>
> Cc: Thinh Nguyen <Thinh.Nguyen@...opsys.com>;
> gregkh@...uxfoundation.org; linux-usb@...r.kernel.org; linux-
> kernel@...r.kernel.org; rosa.pila@...sung.com; dev.tailor@...sung.com;
> faraz.ata@...sung.com; muhammed.ali@...sung.com;
> selvarasu.g@...sung.com
> Subject: Re: [PATCH] usb: dwc3: Allow usb role swich control from userspace
> 
> On Mon, Oct 20, 2025, Pritam Manohar Sutar wrote:
> > There is a possibility of user needs for USB mode switching on boards
> > that lack external hardware support for dynamic host/device role
> > detection.
> 
> It's fine to enable this. But base on this change log, it sounds like there's no use
> case at the moment? If there is one, even if it's only for debugging purpose,
> please note it so. I'm hesitant to accept changes base on use case speculation
> only.
> 

Thank you for your comments. 

Yes, a use case exists, especially in automotive, where this feature
is needed to allow user-space applications to switch modes 
(host to device or vice-versa) at runtime during CarPlay or Android
Auto operations.

Can you please confirm updated commit message with usecase added as below? 

"
usb: dwc3: Allow usb role swich control from userspace

There is a possibility of user needs for USB mode switching on boards
that lack external hardware support for dynamic host/device role
detection. This is particularly relevant in automotive applications 
where userspace applications need to switch USB roles (host to device)
at runtime for CarPlay/Android Auto integration.

Add an `allow_userspace_control` flag to handle such cases. When
enabled, it exposes a sysfs attribute that allows userspace to switch
the USB role manually between host and device. This provides flexibility
for platforms that cannot rely on hardware-based mode detection.

The role switch can be done as below
echo host > /sys/class/usb_role/<ADDR>.usb-role-switch/role
echo device > /sys/class/usb_role/<ADDR>.usb-role-switch/role
"

> Thanks,
> Thinh
> 
> >
> > Add an `allow_userspace_control` flag to handle such cases. When
> > enabled, it exposes a sysfs attribute that allows userspace to switch
> > the USB role manually between host and device. This provides
> > flexibility for platforms that cannot rely on hardware-based mode detection.
> >
> > The role switch can be done as below
> > echo host > /sys/class/usb_role/<ADDR>.usb-role-switch/role
> > echo device > /sys/class/usb_role/<ADDR>.usb-role-switch/role
> >
> > Signed-off-by: Pritam Manohar Sutar <pritam.sutar@...sung.com>
> > ---
> >  drivers/usb/dwc3/drd.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/usb/dwc3/drd.c b/drivers/usb/dwc3/drd.c index
> > 4c91240eb429..589bbeb27454 100644
> > --- a/drivers/usb/dwc3/drd.c
> > +++ b/drivers/usb/dwc3/drd.c
> > @@ -515,6 +515,7 @@ static int dwc3_setup_role_switch(struct dwc3 *dwc)
> >  	dwc3_role_switch.set = dwc3_usb_role_switch_set;
> >  	dwc3_role_switch.get = dwc3_usb_role_switch_get;
> >  	dwc3_role_switch.driver_data = dwc;
> > +	dwc3_role_switch.allow_userspace_control = true;
> >  	dwc->role_sw = usb_role_switch_register(dwc->dev,
> &dwc3_role_switch);
> >  	if (IS_ERR(dwc->role_sw))
> >  		return PTR_ERR(dwc->role_sw);
> > --
> > 2.34.1
> >

Thank you,

Regards,
Pritam