linux-kernel - Re: [PATCH v5 3/4] usb: host: xhci-plat: Create platform device for onboard hubs in probe()

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Message-ID: <YCWVU7YCE+WzRx9v@google.com>
Date:   Thu, 11 Feb 2021 12:36:35 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Krzysztof Kozlowski <krzk@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Peter Chen <peter.chen@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Bastien Nocera <hadess@...ess.net>,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        linux-usb@...r.kernel.org, Mathias Nyman <mathias.nyman@...el.com>
Subject: Re: [PATCH v5 3/4] usb: host: xhci-plat: Create platform device for
 onboard hubs in probe()

Hi Stephen,

On Thu, Feb 11, 2021 at 11:14:39AM -0800, Stephen Boyd wrote:
> Quoting Matthias Kaehlcke (2021-02-10 14:20:18)
> > 
> > On Wed, Feb 10, 2021 at 10:06:45PM +0100, Krzysztof Kozlowski wrote:
> > > 
> > > This looks hackish... what if later we have something else than hub?
> > > Another if()?
> > > 
> > > What if hub could be connected to something else than XHCI controller?
> > 
> > In earlier versions this was standalone driver, which was more flexible and
> > didn't require cooperation from the XHCI driver:
> > 
> > https://lore.kernel.org/patchwork/patch/1313001/
> > 
> > Rob Herring raised objections about the DT bindings, since the USB hub would be
> > represented twice in the DT, once in the USB hierachry (with an explicit node or
> > implicitly) plus a node for the platform device for the new driver:
> > 
> > https://lore.kernel.org/patchwork/patch/1305395/
> > https://lore.kernel.org/patchwork/patch/1313000/
> > 
> > Alan Stern suggested to create the platform device in the XHCI platform driver:
> > 
> > https://lore.kernel.org/patchwork/patch/1313000/#1510227
> > 
> > I wasn't super happy about involving xhci-plat, but at least the code is minimal
> > and all the device specific stuff is handled by the onboard_usb_hub driver.
> > 
> > If you have better suggestions that might satisfy all parties please let us
> > know :)
> > 
> 
> Is it possible to use the graph binding to connect the USB controller on
> the SoC to the port on the hub? Then the hub would be a standalone node
> at the root of DT connected to the USB controller (or phy) and xhci code
> could probe the firmware to see if there's a graph connection downstream
> that is a powered hub like this. I didn't see this idea mentioned in the
> previous discussions, but maybe I missed it.

Thanks for bringing this up. I'm not sure I completely understand your
suggestion, but in general it seems a direction that could be worth
exploring.

I think something like the following should work even without requiring
cooperation from the XHCI code:

onboard-usb-hub {
    compatible = “realtek,rts5411”, “onboard_usb_hub”;
    #address-cells = <1>;
    #size-cells = <0>;

    vdd-supply = <&pp3300_hub>;

    port@0 {
        reg = <0>;

        rts5411_3_0: endpoint {
            // should not be needed
            remote-endpoint = <&usb_1_dwc3_port1>;
        };
    };

    port@1 {
        reg = <1>;

        rts5411_2_0: endpoint {
            // should not be needed
            remote-endpoint = <&usb_1_dwc3_port2>;
        };
    };
};

&usb_1_dwc3 {
    dr_mode = "host";
    #address-cells = <1>;
    #size-cells = <0>;

    port@1 {
        reg = <1>;

        usb_1_dwc3_port1: endpoint {
            remote-endpoint = <&rts5411_3_0>;
        };
    };

    port@2 {
        reg = <2>;

        usb_1_dwc3_port2: endpoint {
            remote-endpoint = <&rts5411_2_0>;
        };
    };
};

That looks like an actual description of the hardware, without multiple DT
nodes for the hub.

The USB part of the onboard_hub driver could determine the platform device
from the remote endpoint and register the USB device with it.