linux-kernel - Re: USB cdc-acm driver: break and command

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Message-ID: <c9d07302-37f0-4f0d-8669-094aa6fc2450@suse.com>
Date: Thu, 17 Jul 2025 13:28:05 +0200
From: Oliver Neukum <oneukum@...e.com>
To: "H. Peter Anvin" <hpa@...or.com>, Oliver Neukum <oneukum@...e.com>
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 gregkh@...uxfoundation.org, Jiri Slaby <jirislaby@...nel.org>,
 linux-serial@...r.kernel.org
Subject: Re: USB cdc-acm driver: break and command

On 16.07.25 19:49, H. Peter Anvin wrote:
> On 2025-07-16 09:17, Oliver Neukum wrote:

>> Understood. It still seems dirty to me. If you want to send strings to a
>> device
>> the proper way is to use a device node and write().
>>   
> 
> There is definitely something to be said for that; or at least a file
> descriptor.  We do have cases in the kernel -- notably opening the pts
> corresponding to a ptmx file descriptor -- that do that sort of
> "auxiliary open" kind of thing.
> 
> The big question is how that interacts with the rest of the ACM driver,
> as well as all the lifetime issues you mentioned elsewhere.

It would seem to me that CDC already has something very similar in form
of CDC-WDM. If acm_probe() can call tty_port_register_device(), it can also
register a secondary character device. Or are you worried about how to tell
user space which devices belong together?

	Regards
		Oliver