[PATCH] drm: rcar-du: calculate DPLLCR to be more small jitter

[Kuninori Morimoto]

Hi

I noticed 1 typo, 1 bug on this patch.
I will post v2 patch

> From: Kuninori Morimoto <kuninori.morimoto.gx at renesas.com>
> 
> In general, PLL has VCO (= Voltage controlled oscillator),
> one of the very important electronic feature called as "jitter"
> is related to this VCO.
> In academic generalism, VCO should be maximum to be more small jitter.
> In high frequency clock, jitter will be large impact.
> Thus, selecting Hi VCO is general theory.
> 
>    fin                                 fvco        fout      fclkout
> in --> [1/M] --> |PD| -> [LPF] -> [VCO] -> [1/P] -+-> [1/FDPLL] -> out
>              +-> |  |                             |
>              |                                    |
>              +-----------------[1/N]<-------------+
> 
> 	fclkout = fvco / P / FDPLL -- (1)
> 
> In PD, it will loop until fin/M = fvco/P/N
> 
> 	fvco = fin * P *  N / M -- (2)
> 
> (1) + (2) indicates, fclkout = fin * N / M / FDPLL
> In this device, N = (n + 1), M = (m + 1), P = 2, thus
> 
> 	fout = fin * (n + 1) / (m + 1) / FDPLL
> 
> This is the datasheet formula.
> One note here is that it should be 2000 < fvco < 4096MHz
> To be smaller jitter, fvco should be maximum,
> in other words, N as large as possible, M as small as possible driver
> should select. Here, basically M=1.
> This patch do it.
> 
> Reported-by: HIROSHI INOSE <hiroshi.inose.rb at renesas.com>
> Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx at renesas.com>
> ---
>  drivers/gpu/drm/rcar-du/rcar_du_crtc.c | 36 ++++++++++++++++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> index b492063..45540fe 100644
> --- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> +++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> @@ -125,8 +125,40 @@ static void rcar_du_dpll_divider(struct rcar_du_crtc *rcrtc,
>  	unsigned int m;
>  	unsigned int n;
>  
> -	for (n = 39; n < 120; n++) {
> -		for (m = 0; m < 4; m++) {
> +	/*
> +	 *   fin                                 fvco        fout       fclkout
> +	 * in --> [1/M] --> |PD| -> [LPF] -> [VCO] -> [1/P] -+-> [1/FDPLL] -> out
> +	 *              +-> |  |                             |
> +	 *              |                                    |
> +	 *              +-----------------[1/N]<-------------+
> +	 *
> +	 *	fclkout = fvco / P / FDPLL -- (1)
> +	 *
> +	 * fin/M = fvco/P/N
> +	 *
> +	 *	fvco = fin * P *  N / M -- (2)
> +	 *
> +	 * (1) + (2) indicates
> +	 *
> +	 *	fclkout = fin * N / M / FDPLL
> +	 *
> +	 * NOTES
> +	 *	N = (n + 1), M = (m + 1), P = 2
> +	 *	2000 < fvco < 4096Mhz
> +	 *	Basically M=1
> +	 *
> +	 * To be small jitter,
> +	 * N : as large as possible
> +	 * M : as small as possible
> +	 */
> +	for (m = 0; m < 4; m++) {
> +		for (n = 119; n > 40; n--) {
> +			unsigned long long fvco = input * 2 * (n + 1) / (m + 1);
> +
> +			if ((fvco < 2000) ||
> +			    (fvco > 4096000000ll))
> +				continue;
> +
>  			for (fdpll = 1; fdpll < 32; fdpll++) {
>  				unsigned long output;
>  
> -- 
> 1.9.1
>