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Subject[PATCH v4 2/2] drm: rcar-du: calculate DPLLCR to be more small jitter
Date

From: Kuninori Morimoto <kuninori.morimoto.gx@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 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>
---
v3 -> v4

- 2000 -> 2kHz

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..574854a 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
+ * 2kHz < 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
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