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

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Date:   Wed, 30 Sep 2020 14:19:32 -0500
From:   Rob Herring <robh@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Alan Stern <stern@...land.harvard.edu>,
        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 Wed, Sep 30, 2020 at 1:00 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> > On Wed, Sep 30, 2020 at 7:44 AM Rob Herring <robh@...nel.org> wrote:
> > >
> > > We already have hubs in DT. See [1][2][3][4]. What's new here?
>
> After I sent my response I kept thinking about this and I realized
> that I have prior art I can point out too!  :-)  Check out
> "smsc,usb3503a".  That is describing a USB hub too and, at least on
> "exynos5250-spring.dts" is is a top level node.  Since "smsc,usb3503a"
> can be optionally connected to an i2c bus too, it could be listed
> under an i2c controller as well (I believe it wasn't hooked up to i2c
> on spring).
>
> Interestingly enough, the USB Hub that Matthias is trying to add
> support for can _also_ be hooked up to i2c.  We don't actually have
> i2c hooked up on our board, but conceivably it could be.  Presumably,
> if i2c was hooked up, we would have no other choice but to represent
> this chip as several device tree nodes: at least one under the i2c
> controller and one (or two) under the USB controller.  Just because
> (on this board) i2c isn't hooked up doesn't change the fact that there
> is some extra control logic that could be represented in its own
> device tree node.  To me, this seems to give extra evidence that the
> correct way to model this device in device tree is with several nodes.
>
> I'll point out that on "exynos5250-spring.dts" we didn't have to solve
> the problem that Matthias is trying to solve here because we never
> actually supported waking up from USB devices there.  Thus the
> regulator for the hub on spring can be unconditionally powered off in
> suspend.  On newer boards we'd like to support waking up from USB
> devices but also to save power if no wakeup devices are plugged into
> USB.  In order to achieve this we need some type of link from the
> top-level hub device to the actual USB devices that were enumerated.

Yes, in a prior version I mentioned we already had 2 ways to describe
hubs. I view this as a 3rd way.

There's prior art in how we reference an i2c bus for a slave device
that's already on another bus. That's 'i2c-bus' and 'ddc-i2c-bus'. But
that's not really this case.

Rob