linux-kernel - Re: [PATCH] usb: core: support interface node for simple USB devices

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Message-ID: <e59623a7-5438-4a8a-b3c0-13cf7e36baa1@leica-geosystems.com>
Date: Thu, 23 Jan 2025 16:33:54 +0000
From: POPESCU Catalin <catalin.popescu@...ca-geosystems.com>
To: Johan Hovold <johan@...nel.org>
CC: "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"m.felsch@...gutronix.de" <m.felsch@...gutronix.de>,
	GEO-CHHER-bsp-development <bsp-development.geo@...ca-geosystems.com>
Subject: Re: [PATCH] usb: core: support interface node for simple USB devices

On 23/01/2025 16:20, Johan Hovold wrote:
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> On Thu, Jan 23, 2025 at 02:01:00PM +0000, POPESCU Catalin wrote:
>> On 23/01/2025 12:15, Johan Hovold wrote:
>>> On Wed, Jan 22, 2025 at 02:47:32PM +0100, Catalin Popescu wrote:
>>>> A simple usb device has a single configuration and a single interface
>>>> and is considered as a "combined node" when defined in the devicetree.
>>>> If available, its interface node is simply ignored which is a problem
>>>> whenever the interface node has subnodes. To prevent that from happening
>>>> first check for any subnode and ignore the interface node only if no
>>>> subnode was found.
>>>>
>>>> Example: FTDI chip FT234XD that has only one UART interface which is
>>>> being used as a serdev by other driver.
>>>>
>>>> device@1 {
>>>>         compatible = "usb403,6015";
>>>>         reg = <1>;
>>>>
>>>>         #address-cells = <2>;
>>>>         #size-cells = <0>;
>>>>
>>>>         interface@0 {
>>>>                 compatible = "usbif403,6015.config1.0";
>>> Your example makes no sense since if this is the only interface then the
>>> interface node should not be here.
>> That's the problem my patch is trying to address ...
>> Why is it OK to describe multiple interfaces and it is not OK to
>> describe the interface of a simple USB device ?
> It's part of the spec. See commit 1a7e3948cb9f ("USB: add device-tree
> support for interfaces") and its reference to "Open Firmware Recommended
> Practice: Universal Serial Bus Version 1".
I see, thanks a lot for the clarification.
BR,
> Johan