linux-kernel - Re: [PATCH] staging: s5k3e2fx.c: reduce complexity by factoring

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Date:	Wed, 9 Dec 2009 11:57:00 +0200
From:	Dan Carpenter <error27@...il.com>
To:	Justin Madru <jdm64@...ab.com>
Cc:	gregkh@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: s5k3e2fx.c: reduce complexity by factoring

On Sat, Dec 05, 2009 at 06:03:20PM -0800, Justin Madru wrote:
> I was style fixing some code when I ran into this code.
> It seems like the code could be reduced, by factoring the expression.
> But this results in a very simple expression.
> 
> Am I assuming something wrong? Or is this a bug in the original code?
> This doesn't look right because the assignment of s_move[i] has no mention
> of the loop counter.
> 
> Justin
> 
> ------
> 
> the code was looping, seting s_move[i] to the following calculations
> 
> if (actual_step >= 0)
>         s_move[i] = ((((i + 1) * gain + 0x200) - (i * gain + 0x200)) / 0x400);
> else
>         s_move[i] = ((((i + 1) * gain - 0x200) - (i * gain - 0x200)) / 0x400);
> 
> but, by factoring the expression, it can be shown that they both reduce
> to a very simple expression:
> 
>         s_move[i] = gain / 0x400;
> 

Heh.

Looks like an improvement to me.

If gain were 16 bits instead of 32 I would think the "* 0x400 / 0x400" was 
designed to mask out the highest 10 bits.

Acked-by:  Dan Carpenter <error27@...il.com>

regards,
dan carpenter

> Signed-off-by: Justin Madru <jdm64@...ab.com>
> ---
>  drivers/staging/dream/camera/s5k3e2fx.c |   10 +++-------
>  1 files changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/staging/dream/camera/s5k3e2fx.c b/drivers/staging/dream/camera/s5k3e2fx.c
> index f0e49be..d205a2d 100644
> --- a/drivers/staging/dream/camera/s5k3e2fx.c
> +++ b/drivers/staging/dream/camera/s5k3e2fx.c
> @@ -1092,14 +1092,10 @@ static int32_t s5k3e2fx_move_focus(int direction, int32_t num_steps)
>  
>  	actual_step = step_direction * (int16_t)num_steps;
>  	pos_offset = init_code + s5k3e2fx_ctrl->curr_lens_pos;
> -	gain = actual_step * 0x400 / 5;
> +	gain = actual_step / 5;
>  
> -	for (i = 0; i <= 4; i++) {
> -		if (actual_step >= 0)
> -			s_move[i] = ((((i+1)*gain+0x200) - (i*gain+0x200))/0x400);
> -		else
> -			s_move[i] = ((((i+1)*gain-0x200) - (i*gain-0x200))/0x400);
> -	}
> +	for (i = 0; i <= 4; i++)
> +		s_move[i] = gain;
>  
>  	/* Ring Damping Code */
>  	for (i = 0; i <= 4; i++) {
> -- 
> 1.6.5.4
> 
> --
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