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

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Date:	Sun, 27 Mar 2011 14:30:24 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>
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

>   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.

What hardware was it tested on ?

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



> 
> 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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