Subject: Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming calculation

From: Wolfgang Grandegger <[email-masked]>
Date: Thu, 22 Sep 2011 15:07:43 +0200

Hi Oliver,

On 09/22/2011 12:28 PM, Oliver Hartkopp wrote:
> This patch adds support for SJW user settings to not set the synchronization
> jump width (SJW) to 1 in any case when using the in-kernel bittiming
> calculation.
> 
> The ip-tool from iproute2 already supports to pass the user defined SJW
> value. The given SJW value is sanitized with the controller specific sjw_max
> and the calculated tseg2 value. As the SJW can have values up to 4 providing
> this value will lead to the maximum possible SJW automatically. A higher SJW
> allows higher controller oscillator tolerances.
> 
> Signed-off-by: Oliver Hartkopp <[email-masked]>
> 
> ---
> 
> v2: Resend due to wrong mail address header encoding.
> 
> diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
> index 9bf1116..25695bd 100644
> --- a/drivers/net/can/dev.c
> +++ b/drivers/net/can/dev.c
> @@ -150,7 +150,19 @@ static int can_calc_bittiming(struct net_device *dev,
> struct can_bittiming *bt)
>  	bt->prop_seg = tseg1 / 2;
>  	bt->phase_seg1 = tseg1 - bt->prop_seg;
>  	bt->phase_seg2 = tseg2;
> -	bt->sjw = 1;
> +
> +	/* check for sjw user settings */
> +	if (!bt->sjw || !btc->sjw_max)
> +		bt->sjw = 1;
> +	else {
> +		/* bt->sjw is at least 1 -> sanitize upper bound to sjw_max */
> +		if (bt->sjw > btc->sjw_max)
> +			bt->sjw = btc->sjw_max;
> +		/* bt->sjw must not be higher than tseg2 */
> +		if (tseg2 < bt->sjw)
> +			bt->sjw = tseg2;
> +	}
> +

As I already said, I expected "bt->sjw" to be always 0 when
can_calc_bittiming() is called. But the software somehow allows to
preset "bt->sjw", which is not intended as the help of the ip utility shows:

 $ ip link set can0 type can help
   Usage: ip link set DEVICE type can
       [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
       [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
         phase-seg2 PHASE-SEG2 [ sjw SJW ] ]

I actually hesitate to extend can_calc_bittiming(). I suggest using an
external tool to calculate proper bit-timing parameters and set them
with "ip link set DEVICE type can tq ...".

See also:

http://lxr.linux.no/#linux+v3.0.4/Documentation/networking/can.txt#L742

Wolfgang.