linux-kernel - Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs

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Date:   Tue, 6 Oct 2020 10:18:20 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Stephen Boyd <swboyd@...omium.org>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Peter Chen <peter.chen@....com>
Subject: Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete
 onboard USB hubs

On Mon, Oct 05, 2020 at 05:45:10PM -0700, Matthias Kaehlcke wrote:
> I did some prototyping, it seems a binding like this would work for
> case a) or b):
> 
> &usb_1_dwc3 {
>         hub_2_0: hub@1 {
>                 compatible = "usbbda,5411";
>                 reg = <1>;
> 	};
> 
>         hub_3_0: hub@2 {
>                 compatible = "usbbda,411";
>                 reg = <2>;
>                 vdd-supply = <&pp3300_hub>;
> 		companion-hubs = <&hub_2_0>;
>         };
> };
> 
> It still requires specifying both hubs (which reflects the actual wiring).
> Supporting something like "reg = <1 2>" seems more complex due to the need to
> obtain the hub USB device at runtime (a DT node makes that trivial), possibly
> this could be solved by adding new APIs.
> 
> In terms of implementation would I envision to keep a platform driver. This
> would keep the hubby parts out of xhci-plat (except for populating the platform
> devices), support systems with cascaded hubs and provide a device for the sysfs
> attribute.

What will you do if a system has more than one of these power-regulated 
hubs?  That is, how will the user know which platform device handles the 
power control for a particular hub (and vice versa)?  You'd probably 
have to create a pair of symlinks going back and forth in the sysfs 
directories.

Wouldn't it be easier to put the power-control attribute directly in the 
hub's sysfs directory (or .../power subdirectory)?

Alan Stern