linux-kernel - Re: [RFC DO NOT MERGE] tty: n

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Date:	Wed, 08 Jan 2014 14:26:03 +0100
From:	Lars Poeschel <poeschel@...onage.de>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.cz,
	linux-kernel@...r.kernel.org, xiaojin <jin.xiao@...el.com>,
	Ken Mills <ken.k.mills@...el.com>,
	Dirkjan Bussink <d.bussink@...il.com>
Subject: Re: [RFC DO NOT MERGE] tty: n_gsm: support buggy modem

Am Dienstag, 7. Januar 2014, 16:51:37 schrieb Alan Cox:
> > > modem status command, but this DV bit is not set. With this patch the
> > > modem
> > > and the whole mux is working fine. Another way makeing all this work is
> > > setting the carrier_raised function to NULL. The tty code assumes that
> > > everything is alright then.
> 
> Which means your modem has decided it doesn't yet have a carrier. Do you
> get a carrier after ATD completes with a CONNECTED message ?
> 
> (you can open with O_NDELAY to avoid blocking on the open)

Oh! Thank you! This does indeed bring up the channel. After opening 
nonblocking and writing ATD *99***1# to a muxed channel the modem sends a 
modem status command message with DV bit set.
This does in fact mean, I can only use the mux in nonblocking mode. Does this 
make sense ? Why can't I use a muxed channel with blocking IO ? I could do AT 
commands over the mux channel and the DV bit is not set (and thus TIOCM_CD is 
not set) and the modem (and the mux line discipline) is fine with it.

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