[PATCH v3 2/2] drm: rcar-du: calculate DPLLCR to be more small jitter

[Kuninori Morimoto]

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, FDPLL = (fdpll + 1).

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

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>
---
v2 -> v3

 - uses "* 1000" for number
 - uses "xx000U" for number
 - uses finnm to avoid duplicate calculation

 drivers/gpu/drm/rcar-du/rcar_du_crtc.c | 58 +++++++++++++++++++++++++++++++---
 1 file changed, 54 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
index 6820461f..bb5ead6 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
@@ -125,13 +125,63 @@ 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)
+	 *	FDPLL	: (fdpll + 1)
+	 *	P	: 2
+	 *	2000 < fvco < 4096Mhz
+	 *
+	 * To be small jitter,
+	 * N : as large as possible
+	 * M : as small as possible
+	 */
+	for (m = 0; m < 4; m++) {
+		for (n = 119; n > 38; n--) {
+			/*
+			 * NOTE:
+			 *
+			 * This code is assuming "used" from 64bit CPU only,
+			 * not from 32bit CPU. But both can compile correctly
+			 */
+
+			/*
+			 *	fvco	= fin * P *  N / M
+			 *	fclkout	= fin      * N / M / FDPLL
+			 *
+			 * To avoid duplicate calculation, let's use below
+			 *
+			 *	finnm	= fin * N / M
+			 *	fvco	= finnm * P
+			 *	fclkout	= finnm / FDPLL
+			 */
+			unsigned long finnm = input * (n + 1) / (m + 1);
+			unsigned long fvco  = finnm * 2;
+
+			if (fvco < 2000 || fvco > 4096 * 1000 * 1000U)
+				continue;
+
 			for (fdpll = 1; fdpll < 32; fdpll++) {
 				unsigned long output;
 
-				output = input * (n + 1) / (m + 1)
-				       / (fdpll + 1);
+				output = finnm / (fdpll + 1);
 				if (output >= 400 * 1000 * 1000)
 					continue;
 
-- 
1.9.1