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

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Message-ID: <f63f2b91-a217-41fb-bbbb-1810d98838fd@suse.com>
Date: Thu, 17 Jul 2025 13:32:01 +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 19:30, H. Peter Anvin wrote:

> ERRORS
> 
>      The tcsendbreak() function shall fail if:
> 
>      [EBADF]
>          The fildes argument is not a valid file descriptor.
>      [EIO]
>          The process group of the writing process is orphaned, the
> calling thread is not blocking SIGTTOU, and the process is not ignoring
> SIGTTOU.
>      [ENOTTY]
>          The file associated with fildes is not a terminal.

I would take this as meaning that we cannot just return -EBUSY or -EWOULDBLOCK.
Hence the generic layer would need to implement some sort of waiting
logic.

> That's why I said if that is what is needed, it really belongs in the
> tty core.  That's where the current internal delay is, after all.

Good. Don't get me wrong. I'd love to do this more efficiently,
but the current API is less than optimal.

	Regards
		Oliver