drm/bridge: tc358768: Cleanup PLL calculations

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.
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> # Asus TF700T
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-tc358768-v4-5-31725f008a50@ideasonboard.com

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,