[PATCH 19/25] serial: sh-sci: Add support for optional external (H)SCK input

[Geert Uytterhoeven]

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

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: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
---
 drivers/tty/serial/sh-sci.c | 63 ++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 12800e52f41953dc..f88aac684ed1e3b6 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/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 +1925,36 @@ 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;
+	}
+
+	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,
@@ -2005,7 +2036,7 @@ 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;
@@ -2043,10 +2074,26 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
 	if (!baud)
 		goto done;
 
+	/* Optional 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;
+		}
+	}
+
 	/* Functional Clock and 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;
@@ -2060,14 +2107,20 @@ done:
 
 	sci_port_enable(s);
 
+	/* Program the optional External Baud Rate Generator (BRG) first */
+	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)
@@ -2303,10 +2356,14 @@ 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)
-- 
1.9.1

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