Re: [PATCH 1/2] mac80211: fix sign error in pid controller

[Mattias Nissler <mattias.nissler@gmx.de>]

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On Sat, 2010-01-16 at 14:36 -0500, Bob Copeland wrote:
> While testing the pid rate controller in mac80211_hwsim, I noticed
> that once the controller reached 54 Mbit rates, it would fail to
> lower the rate when necessary.  The debug log shows:
> 
> 1945 186786 pf_sample 50 3534 3577 50
> 
> My interpretation is that the fixed point scaling of the target
> error value (pf) is incorrect: the error value of 50 compared to
> a target of 14 case should result in a scaling value of
> (14-50) = -36 * 256 or -9216, but instead it is (14 * 256)-50, or
> 3534.
> 
> Correct this by doing fixed point scaling after subtraction.
> 
> Cc: stefano.brivio@polimi.it
> Cc: mattias.nissler@gmx.de
> Signed-off-by: Bob Copeland <me@bobcopeland.com>

I took a quick look and found that you're right. pf is an unscaled
value, so we should subtract it before scaling.

Acked-by: Mattias Nissler <mattias.nissler@gmx.de>

> ---
> 
> Mattias / Stefano, please advise if I missed something.
> 
>  net/mac80211/rc80211_pid_algo.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/mac80211/rc80211_pid_algo.c b/net/mac80211/rc80211_pid_algo.c
> index 699d3ed..29bc4c5 100644
> --- a/net/mac80211/rc80211_pid_algo.c
> +++ b/net/mac80211/rc80211_pid_algo.c
> @@ -190,7 +190,7 @@ static void rate_control_pid_sample(struct rc_pid_info *pinfo,
>  	rate_control_pid_normalize(pinfo, sband->n_bitrates);
>  
>  	/* Compute the proportional, integral and derivative errors. */
> -	err_prop = (pinfo->target << RC_PID_ARITH_SHIFT) - pf;
> +	err_prop = (pinfo->target - pf) << RC_PID_ARITH_SHIFT;
>  
>  	err_avg = spinfo->err_avg_sc >> pinfo->smoothing_shift;
>  	spinfo->err_avg_sc = spinfo->err_avg_sc - err_avg + err_prop;


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