linux-kernel - Re: [PATCH] USB: cdc-acm: expose serial close_delay and closing

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Message-ID: <d1658254-b65b-4e2e-8c22-45008374d6eb@rowland.harvard.edu>
Date:   Thu, 24 Aug 2023 21:53:28 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Simon Arlott <simon@...iron.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH] USB: cdc-acm: expose serial close_delay and closing_wait
 in sysfs

On Fri, Aug 25, 2023 at 01:46:15AM +0200, Oliver Neukum wrote:
> 
> 
> On 24.08.23 20:02, Simon Arlott wrote:
> > The ioctl can't be used without opening and closing the tty, which has
> > side effects. On open() it'll raise DTR/RTS and activate it. For cdc-acm
> > that will indicate to the device that the serial port has been opened
> > which will be visible to the software running on the USB device. On
> > close() it'll be delayed by the close_delay if any process is currently
> > doing a blocking open() and there's no carrier, then the closing_wait
> > time if there's been any incomplete transmitted data (by any process).
> 
> And that is an issue of the generic serial layer.

Is it feasible to add a sysfs attribute for ttys or the serial layer to 
control the side effect of opening (avoid raising DTR/RTS)?  If that 
could be done, a program could use the existing ioctl to set close_delay 
and closing_wait to 0 with no penalties.

This would be racy, but for the purposes of udev that shouldn't matter 
much.

Alan Stern