From: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Date: Mon, 18 Dec 2017 00:35:56 +0000
Subject: drm: rcar-du: Calculate DPLLCR to be more small jitter
Git-commit: 0bc69592abc870f45d82e72c40e44a913d5b90c0
Patch-mainline: v4.17-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

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, FDPLL = (fdpll + 1).

	fclkout = fin * (n + 1) / (m + 1) / (fdpll + 1)

This is the datasheet formula.
One note here is that it should be 2kHz < 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@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[Small clarifications in comments, renamed finnm to fout]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/rcar-du/rcar_du_crtc.c |   49 ++++++++++++++++++++++++++++++---
 1 file changed, 45 insertions(+), 4 deletions(-)

--- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
@@ -125,13 +125,54 @@ static void rcar_du_dpll_divider(struct
 	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)
+	 *	FDPLL	: (fdpll + 1)
+	 *	P	: 2
+	 *	2kHz < fvco < 4096MHz
+	 *
+	 * To minimize the jitter,
+	 * N : as large as possible
+	 * M : as small as possible
+	 */
+	for (m = 0; m < 4; m++) {
+		for (n = 119; n > 38; n--) {
+			/*
+			 * This code only runs on 64-bit architectures, the
+			 * unsigned long type can thus be used for 64-bit
+			 * computation. It will still compile without any
+			 * warning on 32-bit architectures.
+			 *
+			 * To optimize calculations, use fout instead of fvco
+			 * to verify the VCO frequency constraint.
+			 */
+			unsigned long fout = input * (n + 1) / (m + 1);
+
+			if (fout < 1000 || fout > 2048 * 1000 * 1000U)
+				continue;
+
 			for (fdpll = 1; fdpll < 32; fdpll++) {
 				unsigned long output;
 
-				output = input * (n + 1) / (m + 1)
-				       / (fdpll + 1);
+				output = fout / (fdpll + 1);
 				if (output >= 400 * 1000 * 1000)
 					continue;