[PATCH 05/10] serial: max310x: improve max310x_set_ref_clk() efficiency

[Hugo Villeneuve]

From: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>

The same basic code is duplicated for each of the four PLL multiplier
values, so simplify and streamline it. Doing so allows us to avoid
some unnecessary (fdiv * factor) multiplications for some fdiv values and
replace the second "if" statement with an "else if" and avoid a useless
comparison.

Signed-off-by: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 91 +++++++++++++++++++++++++-------------------
 1 file changed, 52 insertions(+), 39 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index f689958736f71b15b7c113214cda60db4aa7c593..1cd7632831a2826ddd42127b1cde2a10dfa412f0 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -258,6 +258,17 @@
 #define MAX14830_BRGCFG_CLKDIS_BIT	(1 << 6) /* Clock Disable */
 #define MAX14830_REV_ID			(0xb0)
 
+struct max310x_clk_config_t {
+	u8 prediv; /* Predivider */
+	u8 pll_mult; /* PLL multiplier */
+	unsigned int fref; /*
+			    * Reference clock for fractional baud rate generator:
+			    *   PLL enabled:  (freq / prediv) x pll_mult
+			    *   PLL disabled: freq
+			    */
+	unsigned int err; /* Computed error for selected parameters */
+};
+
 struct max310x_if_cfg {
 	int (*extended_reg_enable)(struct device *dev, bool enable);
 	u8 rev_id_offset;
@@ -545,70 +556,72 @@ static int max310x_set_baud(struct uart_port *port, int baud)
 	return (16*port->uartclk) / (c*(16*div + frac));
 }
 
-static int max310x_update_best_err(unsigned int f, unsigned int *besterr)
+static void max310x_try_cfg(unsigned int fdiv, u8 div, u8 pll_mult,
+			    struct max310x_clk_config_t *cfg)
 {
+	unsigned int fmul = fdiv * pll_mult;
+	unsigned int err;
+
 	/* Use high-enough baudrate to calculate error */
-	unsigned int err = f % (460800 * 16);
+	err = fmul % (460800 * 16);
 
-	if (*besterr > err) {
-		*besterr = err;
-		return 0;
+	if (cfg->err > err) {
+		cfg->err = err;
+		cfg->pll_mult = pll_mult;
+		cfg->prediv = div;
+		cfg->fref = fmul;
 	}
+}
 
-	return 1;
+static u8 max310x_pll_mult_to_id(u8 pll_mult)
+{
+	switch (pll_mult) {
+	case 144:	return 3;
+	case 96:	return 2;
+	case 48:	return 1;
+	case 6:
+	default:	return 0;
+	}
 }
 
 static int max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
 			       unsigned int freq, unsigned int *fref, bool xtal)
 {
-	unsigned int div, clksrc, pllcfg = 0;
-	unsigned int besterr = UINT_MAX;
-	unsigned int fdiv, fmul, bestfreq = freq;
+	unsigned int div, fdiv, clksrc;
+	struct max310x_clk_config_t cfg;
+
+	cfg.err = UINT_MAX;
+	cfg.prediv = 0;
+	cfg.fref = freq;
 
 	/* First, update error without PLL */
-	max310x_update_best_err(freq, &besterr);
+	max310x_try_cfg(freq, 1, 1, &cfg);
 
 	/* Try all possible PLL dividers */
-	for (div = 1; (div <= 63) && besterr; div++) {
+	for (div = 1; (div <= 63) && cfg.err; div++) {
 		fdiv = DIV_ROUND_CLOSEST(freq, div);
 
-		/* Try multiplier 6 */
-		fmul = fdiv * 6;
 		if ((fdiv >= 500000) && (fdiv <= 800000))
-			if (!max310x_update_best_err(fmul, &besterr)) {
-				pllcfg = (0 << 6) | div;
-				bestfreq = fmul;
-			}
-		/* Try multiplier 48 */
-		fmul = fdiv * 48;
-		if ((fdiv >= 850000) && (fdiv <= 1200000))
-			if (!max310x_update_best_err(fmul, &besterr)) {
-				pllcfg = (1 << 6) | div;
-				bestfreq = fmul;
-			}
-		/* Try multiplier 96 */
-		fmul = fdiv * 96;
+			max310x_try_cfg(fdiv, div, 6, &cfg);   /* PLL x6 */
+		else if ((fdiv >= 850000) && (fdiv <= 1200000))
+			max310x_try_cfg(fdiv, div, 48, &cfg);  /* PLL x48 */
+
 		if ((fdiv >= 425000) && (fdiv <= 1000000))
-			if (!max310x_update_best_err(fmul, &besterr)) {
-				pllcfg = (2 << 6) | div;
-				bestfreq = fmul;
-			}
-		/* Try multiplier 144 */
-		fmul = fdiv * 144;
+			max310x_try_cfg(fdiv, div, 96, &cfg);  /* PLL x96 */
+
 		if ((fdiv >= 390000) && (fdiv <= 667000))
-			if (!max310x_update_best_err(fmul, &besterr)) {
-				pllcfg = (3 << 6) | div;
-				bestfreq = fmul;
-			}
+			max310x_try_cfg(fdiv, div, 144, &cfg); /* PLL x144 */
 	}
 
 	/* Configure clock source */
 	clksrc = MAX310X_CLKSRC_EXTCLK_BIT | (xtal ? MAX310X_CLKSRC_CRYST_BIT : 0);
 
 	/* Configure PLL */
-	if (pllcfg) {
+	if (cfg.prediv) {
+		u8 pll_id = max310x_pll_mult_to_id(cfg.pll_mult);
+
 		clksrc |= MAX310X_CLKSRC_PLL_BIT;
-		regmap_write(s->regmap, MAX310X_PLLCFG_REG, pllcfg);
+		regmap_write(s->regmap, MAX310X_PLLCFG_REG, (pll_id << 6) | cfg.prediv);
 	} else
 		clksrc |= MAX310X_CLKSRC_PLLBYP_BIT;
 
@@ -632,7 +645,7 @@ static int max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
 					     "clock is not stable\n");
 	}
 
-	*fref = bestfreq;
+	*fref = cfg.fref;
 
 	return 0;
 }

-- 
2.47.3