linux-kernel - Re: [PATCH] tty/n_gsm: fix bug in CRC calculation for gsm1 mode

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Date:	Sun, 27 Mar 2011 21:33:00 +0400
From:	Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-serial@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	stable@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/n_gsm: fix bug in CRC calculation for gsm1 mode

On Sun, 27 Mar 2011 14:30:24 +0100
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> >   gsm->received_fcs is not used for GSM1 mode. Thus we put an
> >   additional byte to CRC calculation. As result we get a wrong CRC
> >   and reject incoming frame.
> 
> That much is true except that it was originally tested with gsm encoding
> 1 on various modems for a while and not seen the problem. So I am trying
> to work out why or whether some of the GSM0 patches broke GSM1 somewhere.

as i can see, it's your commit c2f2f0000bb69f067fea12624272e6a58a811702.

Why we need received_fcs variable? We can completely drop it and calculate
fsc in gsm0_receive() and gsm1_receive(). gsm_queue() in this case will
contain only fsc checking code.

> What hardware was it tested on ?

Enfora, Inc. Enabler-III G Modem

> It looks right - probably the code wants pushing into
> gsm0_receive/gsm1_receive appropriately so no special cases leak into
> gsm_queue.

It's possible, see my comment about received_fcs variable above

> 
> 
> > 
> > Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>
> > ---
> >  drivers/tty/n_gsm.c |    8 ++++++--
> >  1 files changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> > index 176f632..4806276 100644
> > --- a/drivers/tty/n_gsm.c
> > +++ b/drivers/tty/n_gsm.c
> > @@ -1658,8 +1658,12 @@ static void gsm_queue(struct gsm_mux *gsm)
> >  
> >  	if ((gsm->control & ~PF) == UI)
> >  		gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf, gsm->len);
> > -	/* generate final CRC with received FCS */
> > -	gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
> > +	if (gsm->encoding == 0){
> > +		/* WARNING: gsm->received_fcs is used for gsm->encoding = 0 only.
> > +		            In this case it contain the last piece of data
> > +		            required to generate final CRC */
> > +		gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
> > +	}
> >  	if (gsm->fcs != GOOD_FCS) {
> >  		gsm->bad_fcs++;
> >  		if (debug & 4)
> 
> 
> -- 
> --
> 	"Alan, I'm getting a bit worried about you."
> 				-- Linus Torvalds
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