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

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Message-ID: <ce54ae11-72bb-4ac7-980b-c1cbc798a209@zytor.com>
Date: Tue, 15 Jul 2025 14:00:10 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: 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: USB cdc-acm driver: break and command

Hi,

I noticed looking at the CDC-ACM driver that it uses the assert/local 
delay/deassert method of sending BREAK.  Given that the CDC model has a 
delay specifier in the command packet, is there any reason not to set 
TTY_DRIVER_HARDWARE_BREAK and sending only one packet?

I'm also wondering if it would make sense to support the 
SEND_ENCAPSULATED_COMMAND and GET_ENCAPSULATED_RESPONSE commands, 
presumably via an ioctl().  I'm not 100% sure because I'm not sure there 
aren't potential security issues.

I'm guessing both of these could be of some use to embedded devices that 
emulate a ttyACM serial port.

	-hpa