linux-kernel - Re: tty_tiocmset() masks out TIOCM_RI and TIOCM

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Message-ID: <20090121153229.4a11cb8e@lxorguk.ukuu.org.uk>
Date:	Wed, 21 Jan 2009 15:32:29 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Adam Bliss <abliss@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Russell King <rmk@...nt.arm.linux.org.uk>,
	Marcel Holtmann <marcel@...tmann.org>
Subject: Re: tty_tiocmset() masks out TIOCM_RI and TIOCM_CD, which breaks
 rfcomm

> TIOCM_CD and TIOCM_RI to the list?  Like this:

CD and RI are inputs which is why they are masked.

> Traditionally, RI (Ring Indicator) and CD (Carrier Detect) were
> typically used for DCE -> DTE communication, so perhaps this is why
> they were left off initially.  However, when Bluetooth is used to

The cables end up doing the conversion so the PC world always sees them
from the one end - even if they are in fact crossed over in the cable.

> It might be appropriate to include other signals as well, such as CTS
> and DSR.  However, these do not seem necessary for rfcomm, since the
> rfcomm driver will interpret a DTR as a DSR and an RTS as a CTS.  My
> main concern is for RI and CD, for which there does not seem to be a
> workaround.

I would have expected the driver to be remapping RI/CD just as it does
for the DTR<->DSR RTS<->CTS cable switch around.

It's certainly possible to fix it, but that would need a double check on
the existing drivers and that passing new suprise bits won't do any harm.
The real question is what should the right behaviour be - and to me that
seems to be to logically flip all the lines not just DTR/DSR RTS/CTS
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