linux-kernel - [PATCH v1 14/16] serial: 8250_dw: Add skip_clk_set

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Date:   Tue, 12 Oct 2021 15:40:25 +0200
From:   Emil Renner Berthing <kernel@...il.dk>
To:     linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-serial@...r.kernel.org
Cc:     Emil Renner Berthing <kernel@...il.dk>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Sagar Kadam <sagar.kadam@...ive.com>,
        Drew Fustini <drew@...gleboard.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Anup Patel <anup.patel@....com>,
        Atish Patra <atish.patra@....com>,
        Matteo Croce <mcroce@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v1 14/16] serial: 8250_dw: Add skip_clk_set_rate quirk

On the StarFive JH7100 SoC the uart core clocks can't be set to exactly
16 * 115200Hz and many other common bitrates. Trying this will only
result in a higher input clock, but low enough that the uart's internal
divisor can't come close enough to the baud rate target. So rather than
try to set the input clock it's better to rely solely on the uart's
internal divisor.

Signed-off-by: Emil Renner Berthing <kernel@...il.dk>
---
 drivers/tty/serial/8250/8250_dw.c | 32 ++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index a3a0154da567..dee26c0909fd 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -51,6 +51,7 @@ struct dw8250_data {
 
 	unsigned int		skip_autocfg:1;
 	unsigned int		uart_16550_compatible:1;
+	unsigned int		skip_clk_set_rate:1;
 };
 
 static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data)
@@ -334,21 +335,23 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	long rate;
 	int ret;
 
-	clk_disable_unprepare(d->clk);
-	rate = clk_round_rate(d->clk, newrate);
-	if (rate > 0) {
-		/*
-		 * Premilinary set the uartclk to the new clock rate so the
-		 * clock update event handler caused by the clk_set_rate()
-		 * calling wouldn't actually update the UART divisor since
-		 * we about to do this anyway.
-		 */
-		swap(p->uartclk, rate);
-		ret = clk_set_rate(d->clk, newrate);
-		if (ret)
+	if (!d->skip_clk_set_rate) {
+		clk_disable_unprepare(d->clk);
+		rate = clk_round_rate(d->clk, newrate);
+		if (rate > 0) {
+			/*
+			 * Premilinary set the uartclk to the new clock rate so
+			 * the clock update event handler caused by the
+			 * clk_set_rate() calling wouldn't actually update the
+			 * UART divisor since we about to do this anyway.
+			 */
 			swap(p->uartclk, rate);
+			ret = clk_set_rate(d->clk, newrate);
+			if (ret)
+				swap(p->uartclk, rate);
+		}
+		clk_prepare_enable(d->clk);
 	}
-	clk_prepare_enable(d->clk);
 
 	p->status &= ~UPSTAT_AUTOCTS;
 	if (termios->c_cflag & CRTSCTS)
@@ -418,6 +421,8 @@ static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 		}
 		if (of_device_is_compatible(np, "marvell,armada-38x-uart"))
 			p->serial_out = dw8250_serial_out38x;
+		if (of_device_is_compatible(np, "starfive,jh7100-uart"))
+			data->skip_clk_set_rate = true;
 
 	} else if (acpi_dev_present("APMC0D08", NULL, -1)) {
 		p->iotype = UPIO_MEM32;
@@ -700,6 +705,7 @@ static const struct of_device_id dw8250_of_match[] = {
 	{ .compatible = "cavium,octeon-3860-uart" },
 	{ .compatible = "marvell,armada-38x-uart" },
 	{ .compatible = "renesas,rzn1-uart" },
+	{ .compatible = "starfive,jh7100-uart" },
 	{ /* Sentinel */ }
 };
 MODULE_DEVICE_TABLE(of, dw8250_of_match);
-- 
2.33.0