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Date: Thu, 17 Sep 2026 15:08:11 +0100
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Alban Bedel <alban.bedel@lht.dlh.de>
Cc: linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
	Johan Hovold <johan@kernel.org>
Subject: Re: [PATCH] USB: serial: Support attaching serdev devices
Message-ID: <2026091756-unmolded-fructose-a018@gregkh>
References: <20260917081103.1917665-1-alban.bedel@lht.dlh.de>
 <2026091739-spousal-linked-2a36@gregkh>
 <20260917153736.600061de@OMT-CWNXR4TFW5-LHT>
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In-Reply-To: <20260917153736.600061de@OMT-CWNXR4TFW5-LHT>

On Thu, Sep 17, 2026 at 03:37:36PM +0200, Alban Bedel wrote:
> On Thu, 17 Sep 2026 09:27:42 +0100
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
> > On Thu, Sep 17, 2026 at 10:11:03AM +0200, Alban Bedel wrote:
> > > Use tty_port_register_device_serdev() to allow attaching serdev
> > > devices to USB serial ports. Also set the ACPI companion device on the
> > > USB serial device to let the serdev matching work on ACPI systems.  
> > 
> > "also" should be a separate patch, right?
> 
> Will do.
> 
> > And I didn't think that serdev could handle devices going away at any
> > point in time, when did that change?  How was this all tested and what
> > ACPI device wants to use this?
> 
> The use case here is an x86 embedded platform with a soldered USB serial
> adapter connecting to a MAX9265 GMSL serializer. As everything is on a
> single PCB there is no question of anything getting disconnected at
> runtime.

You hope :)

> But I now see that I missed that the bus remove also need to be
> adjusted to use tty_port_unregister_device() instead of
> tty_unregister_device().  With that done the serdev device get properly
> taken down when I manually disable the port. With a small fix to the
> serdev core it also comes back up when the port is enabled again.

Please read the archives for why we don't want to do this unless/until
serdev is "fixed" to properly handle dynamic device removals.

> The ACPI part is a custom DSDT overlay that defines the chain
> starting from the USB port. At this level there is no difference to 
> classic UART where serdev are already working.

So acpi now defines USB to serial devices?  Is that new?

thanks,

greg k-h