linux-kernel - Re: [PATCH v2] Bluetooth: rfcomm: fix modem control handling

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Message-ID: <aQNnq02qzD3rPh2b@hovoldconsulting.com>
Date: Thu, 30 Oct 2025 14:27:07 +0100
From: Johan Hovold <johan@...nel.org>
To: Paul Menzel <pmenzel@...gen.mpg.de>
Cc: Luiz Augusto von Dentz <luiz.dentz@...il.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v2] Bluetooth: rfcomm: fix modem control handling

On Wed, Oct 29, 2025 at 09:15:28PM +0100, Paul Menzel wrote:

> Am 23.10.25 um 14:05 schrieb Johan Hovold:
> > The RFCOMM driver confuses the local and remote modem control signals,
> > which specifically means that the reported DTR and RTS state will
> > instead reflect the remote end (i.e. DSR and CTS).
> > 
> > This issue dates back to the original driver (and a follow-on update)
> > merged in 2002, which resulted in a non-standard implementation of
> > TIOCMSET that allowed controlling also the TS07.10 IC and DV signals by
> > mapping them to the RI and DCD input flags, while TIOCMGET failed to
> > return the actual state of DTR and RTS.
> > 
> > Note that the bogus control of input signals in tiocmset() is just
> > dead code as those flags will have been masked out by the tty layer
> > since 2003.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> There is a linux-history git archive [1], if somebody wants dig further. 
> But not relevant for the tag used by the stable folks.

Yeah, that's tree I use.

> Is there any way to test your change, to read DTR and RTS state?

This can be tested using the TIOCMGET and TIOCMSET (or
TIOCMBIS/TIOCMBIC) ioctls. I use a custom c application, and I'm not
aware of any particular application you can you use for testing if
that's what you were after.

Johan