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

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Date:   Mon, 5 Oct 2020 14:59:04 -0500
From:   Rob Herring <robh@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Doug Anderson <dianders@...omium.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 5, 2020 at 2:36 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Mon, Oct 05, 2020 at 12:18:12PM -0700, Matthias Kaehlcke wrote:
> > On Mon, Oct 05, 2020 at 12:15:27PM -0400, Alan Stern wrote:
> > > The conclusion is that we need to have code that is aware of some
> > > detailed needs of a specific device but is not part of the device's
> > > driver.  I'm not sure what the best way to implement this would be.
> >
> > Wouldn't it be possible to load the module when the DT specifies that
> > the device exists? For USB the kernel would need the VID/PID to identify
> > the module, these could be extracted from the compatible string.
>
> Loading a driver module whenever DT says a device exists?  Not a bad
> idea.  I don't know what would be involved, but no doubt it is possible.

MODULE_DEVICE_TABLE mostly as I mentioned in my other reply.

> Note that, except for a few special cases, the kernel identifies the
> appropriate driver for USB hubs not by the VID/PID but instead by the
> device class or interface class.  I suppose the compatible string could
> include that information too?

We can go back to 1998 OpenFirmware and it's already there[1].
'usb,class9' for a hub. There's a few other variations defined.

> > Having the initialization code outside of the driver could lead to code
> > duplication, since the driver might want to power the device down in
> > certain situations (e.g. system suspend).
>
> True.  On the other hand, how common do you think it would be for
> drivers not to want to mess with the power settings?

I think in that case you'd generally want firmware to enable things
and the kernel then does no power control.

We have ~1500 boards using DT and maybe ~10 with USB devices described
in DT. So the whole thing is not common to begin with.

Rob

[1] https://www.devicetree.org/open-firmware/bindings/usb/usb-1_0.ps