Re: [PATCH v2 2/2] usb: dwc3: dwc3-generic-plat: Add Renesas R-Car Gen5 DWC3 xHCI USB controller glue

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Sat, Aug 08, 2026, Marek Vasut wrote:
> On 8/8/26 1:31 AM, Thinh Nguyen wrote:
> 
> Hello Thinh,
> 
> > > The Renesas R-Car Gen5 SoC contains multiple instances of DWC3 USB
> > > controller with glue logic wrapper around them. Extend the generic
> > > DWC3 platform driver with Renesas R-Car Gen5 glue logic specifics.
> > > 
> > > Signed-off-by: Thanh Quan <thanh.quan.xn@renesas.com>
> > > Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
> 
> [...]
> 
> > > V2: Extend dwc3-generic-plat driver instead
> > 
> > Great! This seems to fit in nicely for dwc3-generic-plat.
> 
> Indeed, this is nice.
> 
> [...]
> 
> > > +static int dwc3_renesas_rcar_gen5_init(struct dwc3_generic *dwc3g)
> > > +{
> > > +	struct device *dev = dwc3g->dev;
> > > +	struct platform_device *pdev = to_platform_device(dev);
> > > +	const char *maximum_speed;
> > > +	bool use_usb3_flow;
> > > +	void __iomem *glue;
> > > +	int ret;
> > > +
> > > +	glue = devm_platform_ioremap_resource_byname(pdev, "glue");
> > > +	if (IS_ERR(glue))
> > > +		return PTR_ERR(glue);
> > > +
> > > +	ret = of_property_read_string(dev->of_node, "maximum-speed", &maximum_speed);
> > > +	if (ret)
> > > +		return dev_err_probe(dev, -ENODEV, "Failed to determine maximum speed\n");
> > 
> > We shouldn't overload the generic "maximum-speed" definition.
> > 
> > Can we check for the presence of the "usb3-phy" instead?
> This could be made to work, but I don't think this would work nicely, please
> see below.
> 
> The Gen5 contains four DWC3 controllers, two are USB2-only and two are
> combined USB2+USB3.2 ; the USB2-only controllers each use one PHY, the
> USB2+USB3.2 controllers each use two PHYs -- one USB2 and one USB3 PHY.
> -> The PHYs are always present and always connected inside the SoC, so I
>    believe the PHYs should always be described in the SoC DT, and the
>    DWC3 controller node should always have phandle(s) to its matching
>    PHYs.
> 
> The USB2-only controllers are a non-issue, there we could get away with
> checking for the presence of "usb3-phy" and if this is missing in DT, apply
> the magic code ("use_usb3_flow" is always false).
> 
> It is the USB2+USB3.2 controllers that are more complicated. These
> controllers can be downgraded to USB2-only mode. Currently, this is done via
> this "maximum-speed" DT property, which, if set to "high-speed" or lower
> triggers the application of the magic code. If the "maximum-speed" DT
> property is "super-speed" or "super-speed-plus", the magic code is not
> applied (this is the "use_usb3_flow").
> 
> If we were to opt for checking for presence of "usb3-phy" , then the board
> DTs (which include the SoC DT) for board which would require the USB2+USB3.2
> controller to be downgraded to USB2-only mode would have to adjust the PHY
> phandles of such controller DT node in board DT, which itself does not seem
> right (DT describes the SoC hardware, and the PHY is still in the SoC and
> connected to the controller, please see "->" above).
> 
> That is why using "maximum-speed" is I think a bit better.
> 
> But maybe there is yet another option that I missed ?
> 
> Thank you for your help !
> 

I see.

My concern is that "maximum-speed" is a generic property that describes
the maximum speed the controller is allowed to operate at. Per the
binding, if it is not present the controller should default to its
maximum HW capability.

Would this work for you? (Given that the usb3 phy phandle must be
present)

	if (!has_usb3_phy || maximum_speed <= USB_SPEED_HIGH)
		use_usb3_flow = false;

This way, "maximum-speed" retains its generic meaning as a speed limit
rather than being used by itself to select Renesas specific flow.

BR,
Thinh