serial: sh-sci: Add support for optional external (H)SCK input

Add support for using the SCIx clock pin "(H)SCK" as an external clock
input on (H)SCI(F), providing the sampling clock.

Note that this feature is not yet supported on the select SCIFA variants
that also have it (e.g. sh7723, sh7724, and r8a7740).

On (H)SCIF variants with an External Baud Rate Generator (BRG), the
BRG Clock Select Register must be configured for the external clock.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -79,6 +79,7 @@ enum {
 
 enum SCI_CLKS {
 	SCI_FCK,		/* Functional Clock */
+	SCI_SCK,		/* Optional External Clock */
 	SCI_NUM_CLKS
 };
 
@@ -1924,6 +1925,39 @@ static void sci_shutdown(struct uart_port *port)
 	sci_free_irq(s);
 }
 
+static int sci_sck_calc(struct sci_port *s, unsigned int bps,
+			unsigned int *srr)
+{
+	unsigned long freq = s->clk_rates[SCI_SCK];
+	unsigned int min_sr, max_sr, sr;
+	int err, min_err = INT_MAX;
+
+	if (s->sampling_rate) {
+		/* SCI(F) has a fixed sampling rate */
+		min_sr = max_sr = s->sampling_rate / 2;
+	} else {
+		/* HSCIF has a variable 1/(8..32) sampling rate */
+		min_sr = 8;
+		max_sr = 32;
+	}
+
+	for (sr = max_sr; sr >= min_sr; sr--) {
+		err = DIV_ROUND_CLOSEST(freq, sr) - bps;
+		if (abs(err) >= abs(min_err))
+			continue;
+
+		min_err = err;
+		*srr = sr - 1;
+
+		if (!err)
+			break;
+	}
+
+	dev_dbg(s->port.dev, "SCK: %u%+d bps using SR %u\n", bps, min_err,
+		*srr + 1);
+	return min_err;
+}
+
 /* calculate sample rate, BRR, and clock select */
 static int sci_scbrr_calc(struct sci_port *s, unsigned int bps,
 			  unsigned int *brr, unsigned int *srr,
@@ -2019,7 +2053,7 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
 			    struct ktermios *old)
 {
 	unsigned int baud, smr_val = 0, scr_val = 0, i;
-	unsigned int brr = 255, cks = 0, srr = 15;
+	unsigned int brr = 255, cks = 0, srr = 15, sccks = 0;
 	unsigned int brr1 = 255, cks1 = 0, srr1 = 15;
 	struct sci_port *s = to_sci_port(port);
 	const struct plat_sci_reg *reg;
@@ -2062,10 +2096,26 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
 	 * that gives us the smallest deviation from the desired baud rate.
 	 */
 
+	/* Optional Undivided External Clock */
+	if (s->clk_rates[SCI_SCK] && port->type != PORT_SCIFA &&
+	    port->type != PORT_SCIFB) {
+		err = sci_sck_calc(s, baud, &srr1);
+		if (abs(err) < abs(min_err)) {
+			best_clk = SCI_SCK;
+			scr_val = SCSCR_CKE1;
+			sccks = SCCKS_CKS;
+			min_err = err;
+			srr = srr1;
+			if (!err)
+				goto done;
+		}
+	}
+
 	/* Divided Functional Clock using standard Bit Rate Register */
 	err = sci_scbrr_calc(s, baud, &brr1, &srr1, &cks1);
 	if (abs(err) < abs(min_err)) {
 		best_clk = SCI_FCK;
+		scr_val = 0;
 		min_err = err;
 		brr = brr1;
 		srr = srr1;
@@ -2079,14 +2129,23 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
 
 	sci_port_enable(s);
 
+	/*
+	 * Program the optional External Baud Rate Generator (BRG) first.
+	 * It controls the mux to select (H)SCK or frequency divided clock.
+	 */
+	if (best_clk >= 0 && sci_getreg(port, SCCKS)->size)
+		serial_port_out(port, SCCKS, sccks);
+
 	sci_reset(port);
 
 	uart_update_timeout(port, termios->c_cflag, baud);
 
 	if (best_clk >= 0) {
 		smr_val |= cks;
-		dev_dbg(port->dev, "SMR 0x%x BRR %u SRR %u\n", smr_val, brr,
-			srr);
+		dev_dbg(port->dev,
+			 "SCR 0x%x SMR 0x%x BRR %u CKS 0x%x SRR %u\n",
+			 scr_val, smr_val, brr, sccks, srr);
+		serial_port_out(port, SCSCR, scr_val);
 		serial_port_out(port, SCSMR, smr_val);
 		serial_port_out(port, SCBRR, brr);
 		if (sci_getreg(port, HSSRR)->size)
@@ -2322,10 +2381,14 @@ static int sci_init_clocks(struct sci_port *sci_port, struct device *dev)
 {
 	const char *clk_names[] = {
 		[SCI_FCK] = "fck",
+		[SCI_SCK] = "sck",
 	};
 	struct clk *clk;
 	unsigned int i;
 
+	if (sci_port->cfg->type == PORT_HSCIF)
+		clk_names[SCI_SCK] = "hsck";
+
 	for (i = 0; i < SCI_NUM_CLKS; i++) {
 		clk = devm_clk_get(dev, clk_names[i]);
 		if (PTR_ERR(clk) == -EPROBE_DEFER)