linux-kernel - Re: [PATCH v5 7/7] dt-bindings: usb: add documentation for aspeed usb-vhub

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Date:   Mon, 02 Mar 2020 15:49:07 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Tao Ren <rentao.bupt@...il.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Felipe Balbi <balbi@...nel.org>, linux-aspeed@...ts.ozlabs.org,
        devicetree@...r.kernel.org, Andrew Jeffery <andrew@...id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        openbmc@...ts.ozlabs.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Joel Stanley <joel@....id.au>, taoren@...com,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Colin Ian King <colin.king@...onical.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 7/7] dt-bindings: usb: add documentation for aspeed
 usb-vhub

On Fri, 2020-02-28 at 00:13 -0800, Tao Ren wrote:
> On Fri, Feb 28, 2020 at 02:02:28PM +1100, Benjamin Herrenschmidt wrote:
> > On Thu, 2020-02-27 at 17:05 -0800, Tao Ren wrote:
> > > > Also long run I think best is going to have a child node per downstream
> > > > port, so we create a matching linux struct device. This will make it
> > > > easier to deal with the other device-controller in the ast2600 which is
> > > > basically one of these without a vhub above it.
> > > 
> > > Maybe a dumb question: what would be the proper place to parse the child
> > > node/properties when they are added? For example, in some usb_gadget_ops
> > > callback?
> > 
> > No. What the vhub would do is when it probes, it creates a platform
> > device for each "port" child node that's linked to the DT node.
> > 
> > The driver for the device then attaches to it via standard DT matching
> > and checks if it has a vhub parent or not, and based on that, operates
> > as a vhub child device or a standalone one.
> > 
> > (For example, it might have different functions for EP selection since
> > standalone devices have private EPs rather than a shared pool)
> > 
> > They can both be in the same module or they can be separate modules
> > with cross dependencies.
> > 
> > Cheers,
> > Ben.
> 
> I see. It's to describe these downstream devices (such as configurations
> and according functions) in device tree, which is similar to defining a
> composite device and linking functions/interfaces via configfs. Thanks for
> the clarify.

It's also to make it easier long run to support both the standalone
variant and the vhub variant from the same code base.

Cheers,
Ben.