linux-kernel - Re: [PATCH 0/3] USB-Serial serdev support

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Message-ID: <20240917044948.i2eog4ondf7vna7q@pengutronix.de>
Date: Tue, 17 Sep 2024 06:49:48 +0200
From: Marco Felsch <m.felsch@...gutronix.de>
To: Johan Hovold <johan@...nel.org>
Cc: Rob Herring <robh@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 0/3] USB-Serial serdev support

Hi Johan,

On 24-09-09, Johan Hovold wrote:
> On Wed, Aug 07, 2024 at 04:08:47PM +0200, Marco Felsch wrote:
> > this patchset is based on Johan's patches [1] but dropped the need of
> > the special 'serial' of-node [2].
> 
> That's great that you found and referenced my proof-of-concept patches,
> but it doesn't seem like you tried to understand why this hasn't been
> merged yet.

I'm glad for your input.

> First, as the commit message you refer to below explain, we need some
> way to describe multiport controllers. Just dropping the 'serial' node
> does not make that issue go away.

Sorry for asking but isn't the current OF abstraction [1] enough? As far
as I understood we can describe the whole USB tree within OF. I used [1]
and the this patchset to describe the following hierarchy:

 usb-root -> usb-hub port-1 -> usb-serial interface-0 -> serial
                                                         bt-module

[1] Documentation/devicetree/bindings/usb/usb-device.yaml

> Second, and more importantly, you do not address the main obstacle for
> enabling serdev for USB serial which is that the serdev cannot handle
> hotplugging.

Hotplugging is a good point but out-of-scope IMHO (at least for now)
since the current serdev implementation rely on additional firmware
information e.g OF node to be present. E.g. if the above mentioned setup
would connect the "serial bt-module" directly to the UART port you still
need an OF node to bind the serdev driver. If the node isn't present
user-space would need to do the hci handling.

So from my POV the serdev abstraction is for manufacturers which make
use of "onboard" usb-devices which are always present at the same USB
tree location. Serdev is not made for general purpose USB ports (yet)
where a user can plug-in all types of USB devices.

Regards,
  Marco

> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/log/?h=usb-serial-of
> > [2] https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/commit/?h=usb-serial-of&id=b19239022c92567a6a9ed40e8522e84972b0997f
> 
> Johan
>