Hi
I noticed 1 typo, 1 bug on this patch.
I will post v2 patch
> From: Kuninori Morimoto <kuninori.morimoto...@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...@renesas.com>
> Signed-off-by: Kuninori Morimoto <kuninori.morimoto...@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
>
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