[PATCH 03/11] drm/bridge: tc358768: Cleanup PLL calculations

From: Tomi Valkeinen
Date: Fri Aug 04 2023 - 06:44:56 EST


As is quite common, some of TC358768's PLL register fields are to be
programmed with (value - 1). Specifically, the FBD and PRD, multiplier
and divider, are such fields.

However, what the driver currently does is that it considers that the
formula used for PLL rate calculation is:

RefClk * [(FBD + 1)/ (PRD + 1)] * [1 / (2^FRS)]

where FBD and PRD are values directly from the registers, while a more
sensible way to look at it is:

RefClk * FBD / PRD * (1 / (2^FRS))

and when the FBD and PRD values are written to the registers, they will
be subtracted by one.

Change the driver accordingly, as it simplifies the PLL code.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/bridge/tc358768.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
index b668f77673c3..d5831a1236e9 100644
--- a/drivers/gpu/drm/bridge/tc358768.c
+++ b/drivers/gpu/drm/bridge/tc358768.c
@@ -316,7 +316,7 @@ static int tc358768_calc_pll(struct tc358768_priv *priv,

target_pll = tc358768_pclk_to_pll(priv, mode->clock * 1000);

- /* pll_clk = RefClk * [(FBD + 1)/ (PRD + 1)] * [1 / (2^FRS)] */
+ /* pll_clk = RefClk * FBD / PRD * (1 / (2^FRS)) */

for (i = 0; i < ARRAY_SIZE(frs_limits); i++)
if (target_pll >= frs_limits[i])
@@ -336,19 +336,19 @@ static int tc358768_calc_pll(struct tc358768_priv *priv,
best_prd = 0;
best_fbd = 0;

- for (prd = 0; prd < 16; ++prd) {
- u32 divisor = (prd + 1) * (1 << frs);
+ for (prd = 1; prd <= 16; ++prd) {
+ u32 divisor = prd * (1 << frs);
u32 fbd;

- for (fbd = 0; fbd < 512; ++fbd) {
+ for (fbd = 1; fbd <= 512; ++fbd) {
u32 pll, diff, pll_in;

- pll = (u32)div_u64((u64)refclk * (fbd + 1), divisor);
+ pll = (u32)div_u64((u64)refclk * fbd, divisor);

if (pll >= max_pll || pll < min_pll)
continue;

- pll_in = (u32)div_u64((u64)refclk, prd + 1);
+ pll_in = (u32)div_u64((u64)refclk, prd);
if (pll_in < 4000000)
continue;

@@ -611,7 +611,7 @@ static int tc358768_setup_pll(struct tc358768_priv *priv,
mode->clock * 1000);

/* PRD[15:12] FBD[8:0] */
- tc358768_write(priv, TC358768_PLLCTL0, (prd << 12) | fbd);
+ tc358768_write(priv, TC358768_PLLCTL0, ((prd - 1) << 12) | (fbd - 1));

/* FRS[11:10] LBWS[9:8] CKEN[4] RESETB[1] EN[0] */
tc358768_write(priv, TC358768_PLLCTL1,

--
2.34.1