Re: [PATCH] drm/i915: clean up dsi pll calculation

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, May 07, 2015 at 05:14:17PM +0300, Jani Nikula wrote:
> Improve readability. No functional changes.
> 
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> 
> ---
> 
> Cleaning up old branches... :)
> ---
>  drivers/gpu/drm/i915/intel_dsi_pll.c | 52 ++++++++++++------------------------
>  1 file changed, 17 insertions(+), 35 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dsi_pll.c b/drivers/gpu/drm/i915/intel_dsi_pll.c
> index 3622d0bafdf8..fc139946abee 100644
> --- a/drivers/gpu/drm/i915/intel_dsi_pll.c
> +++ b/drivers/gpu/drm/i915/intel_dsi_pll.c
> @@ -162,53 +162,35 @@ static u32 dsi_clk_from_pclk(u32 pclk, int pixel_format, int lane_count)
>  
>  #endif
>  
> -static int dsi_calc_mnp(u32 dsi_clk, struct dsi_mnp *dsi_mnp)
> +static int dsi_calc_mnp(u32 target_dsi_clk, struct dsi_mnp *dsi_mnp)
>  {
>  	u32 m, n, p;
> -	u32 ref_clk;
> -	u32 error;
> -	u32 tmp_error;
> -	int target_dsi_clk;
> -	int calc_dsi_clk;
> -	u32 calc_m;
> -	u32 calc_p;
> +	u32 ref_clk = 25000;
> +	u32 delta = target_dsi_clk;
> +	u32 calc_m = 0;
> +	u32 calc_p = 0;
>  	u32 m_seed;
>  
> -	/* dsi_clk is expected in KHZ */
> -	if (dsi_clk < 300000 || dsi_clk > 1150000) {
> +	/* target_dsi_clk is expected in KHZ */
> +	if (target_dsi_clk < 300000 || target_dsi_clk > 1150000) {
>  		DRM_ERROR("DSI CLK Out of Range\n");
>  		return -ECHRNG;
>  	}
>  
> -	ref_clk = 25000;
> -	target_dsi_clk = dsi_clk;
> -	error = 0xFFFFFFFF;
> -	tmp_error = 0xFFFFFFFF;
> -	calc_m = 0;
> -	calc_p = 0;
> -
> -	for (m = 62; m <= 92; m++) {
> -		for (p = 2; p <= 6; p++) {
> -			/* Find the optimal m and p divisors
> -			   with minimal error +/- the required clock */
> -			calc_dsi_clk = (m * ref_clk) / p;
> -			if (calc_dsi_clk == target_dsi_clk) {
> -				calc_m = m;
> -				calc_p = p;
> -				error = 0;
> -				break;
> -			} else
> -				tmp_error = abs(target_dsi_clk - calc_dsi_clk);
> -
> -			if (tmp_error < error) {
> -				error = tmp_error;
> +	for (m = 62; m <= 92 && delta; m++) {
> +		for (p = 2; p <= 6 && delta; p++) {
> +			/*
> +			 * Find the optimal m and p divisors with minimal delta
> +			 * +/- the required clock
> +			 */
> +			int calc_dsi_clk = (m * ref_clk) / p;
> +			u32 d = abs(target_dsi_clk - calc_dsi_clk);

'target_dsi_clk - calc_dsi_clk' is going to be unsigned, so this is
still a bit confusing. abs() does stick that into a signed int before
comparing it, so it should just work, but still might be sensible to
make target_dsi_clk signed.

Also perhaps s/u32/unsigned int/ (or maybe just use int) all over?

> +			if (d < delta) {
> +				delta = d;
>  				calc_m = m;
>  				calc_p = p;
>  			}
>  		}
> -
> -		if (error == 0)
> -			break;
>  	}
>  
>  	m_seed = lfsr_converts[calc_m - 62];
> -- 
> 2.1.4
> 
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-- 
Ville Syrjälä
Intel OTC
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