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

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Message-ID: <2363067.TP5jWQXLjM@lem-wkst-02>
Date:	Tue, 07 Jan 2014 17:58:59 +0100
From:	Lars Poeschel <poeschel@...onage.de>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	jslaby@...e.cz, linux-kernel@...r.kernel.org,
	xiaojin <jin.xiao@...el.com>, Ken Mills <ken.k.mills@...el.com>,
	Alan Cox <alan@...ux.intel.com>,
	Dirkjan Bussink <d.bussink@...il.com>
Subject: Re: [RFC DO NOT MERGE] tty: n_gsm: support buggy modem

Am Dienstag, 7. Januar 2014, 06:20:50 schrieb Greg KH:
> On Tue, Jan 07, 2014 at 02:05:26PM +0100, Lars Poeschel wrote:
> > From: Lars Poeschel <poeschel@...onage.de>
> > 
> > This is obiviously a bad patch. Do not merge it!
> > I have a GSM Modem (Quectel M95 Revision: M95AR01A11) that I have a
> > problem
> > with when using the n_gsm mux line discipline. After attaching the line
> > discipline and opening a muxed channel, the open never returns. The
> > problem is that the tty code waits for the TIOCM_CD flag to be set.
> > According to the 3GPP TS 07.10 specification the modem can set this
> > virtual flag by setting the DV bit in it's modem status control message.
> > My modem sends the 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.
> > My question is: What could be the best way to support this modem in
> > mailine
> > kernel ? I do not see a way to detect this buggy modem inside the line
> > discipline and use this patch's method as a work around then. But the
> > detection would only be possible using AT command before the line
> > discipline is attached or using the muxed channel after the mux is set
> > up.
> 
> Adding some n_gsm.c authors to the cc:
> 
> Also, what kernel version are you using?

Sorry, I forgot this.
I am using 3.12. After your mail I tried 3.13-rc7. It behaves exactly the 
same.

> > ---
> > diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> > index c0f76da..a1a6f57 100644
> > --- a/drivers/tty/n_gsm.c
> > +++ b/drivers/tty/n_gsm.c
> > @@ -1061,6 +1061,7 @@ static void gsm_process_modem(struct tty_struct
> > *tty, struct gsm_dlci *dlci,> 
> >  		mlines |= TIOCM_RI;
> >  	
> >  	if (modem & MDM_DV)
> >  	
> >  		mlines |= TIOCM_CD;
> > 
> > +	mlines |= TIOCM_CD;
> > 
> >  	/* Carrier drop -> hangup */
> >  	if (tty) {


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