linux-kernel - Re: [PATCH 0/8] USB: add device-tree support for interfaces

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Date:   Thu, 16 Nov 2017 17:12:49 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Alan Stern <stern@...land.harvard.edu>,
        Peter Chen <peter.chen@....com>, linux-usb@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH 0/8] USB: add device-tree support for interfaces

On Thu, Nov 16, 2017 at 08:43:21AM -0600, Rob Herring wrote:
> On Thu, Nov 09, 2017 at 06:07:15PM +0100, Johan Hovold wrote:
> > This series adds support for representing USB interfaces in device tree
> > by implementing support for "interface nodes" and "combined nodes" from
> > the OF specification.
> > 
> > This is needed to be able to describe non-discoverable properties of
> > permanently attached USB devices and their interfaces such as any
> > i2c-clients connected to a USB-i2c bridge (e.g. the dln2 mfd) or,
> > eventually, devices connected to usb-serial converters (to be used with
> > serdev).
> 
> In the original OF binding, the firmware dynamically generated the tree 
> for the active configuration AIUI. That doesn't really fit for the 
> (mostly) static FDT usage and why we stopped at the device level. So how 
> do we handle multiple configs? Or can we assume that if say the I2C bus 
> is used, then there's only one config and interface that can use it?

Multiple configuration can be used to implement different sets of
functionality. A hypothetical device could have one i2c controller in
one configuration and two in another. Most devices will only have one
configuration though.

A USB interface implements some functionality of the device (and this is
what Linux USB drivers bind to). So even for single-configuration
devices, you need to be able to say which i2c controller (bus) you are
describing.

[ And as a simplification, the combined nodes can be used for most cases
were we only have one configuration with a single interface. ]

Note that a new set of interfaces (in the kernel device model) is
created when a new USB device configuration is selected. These new
interfaces will be associated with any matching device-tree interface
nodes and that these would be distinct from any nodes that matches
another configuration.

So I don't think there's any problem with dealing with the rare cases of
multi-configuration devices. 

Johan