linux-kernel - [PATCH V3 1/2] tty: serial: meson: Add a 12MHz internal clock rate to calculate baud rate in order to meet the baud rate requirements of special BT modules

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Message-ID: <20220422111320.19234-2-yu.tu@amlogic.com>
Date:   Fri, 22 Apr 2022 19:13:19 +0800
From:   Yu Tu <yu.tu@...ogic.com>
To:     <linux-serial@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-amlogic@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>
CC:     Jiri Slaby <jirislaby@...nel.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Yu Tu <yu.tu@...ogic.com>
Subject: [PATCH V3 1/2] tty: serial: meson: Add a 12MHz internal clock rate to calculate baud rate in order to meet the baud rate requirements of special BT modules

A /2 divider over XTAL was introduced since G12A, and is preferred
to be used over the still present /3 divider since it provides much
closer frequencies vs the request baudrate. Especially the BT module
uses 3Mhz baud rate. 8Mhz calculations can lead to baud rate bias,
causing some problems.

Signed-off-by: Yu Tu <yu.tu@...ogic.com>
---
 drivers/tty/serial/meson_uart.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
index 8e59624935af..58bd2723c004 100644
--- a/drivers/tty/serial/meson_uart.c
+++ b/drivers/tty/serial/meson_uart.c
@@ -68,6 +68,7 @@
 #define AML_UART_BAUD_MASK		0x7fffff
 #define AML_UART_BAUD_USE		BIT(23)
 #define AML_UART_BAUD_XTAL		BIT(24)
+#define AML_UART_BAUD_XTAL_DIV2		BIT(27)
 
 #define AML_UART_PORT_NUM		12
 #define AML_UART_PORT_OFFSET		6
@@ -80,6 +81,10 @@ static struct uart_driver meson_uart_driver;
 
 static struct uart_port *meson_ports[AML_UART_PORT_NUM];
 
+struct meson_uart_data {
+	bool has_xtal_div2;
+};
+
 static void meson_uart_set_mctrl(struct uart_port *port, unsigned int mctrl)
 {
 }
@@ -293,13 +298,20 @@ static int meson_uart_startup(struct uart_port *port)
 
 static void meson_uart_change_speed(struct uart_port *port, unsigned long baud)
 {
-	u32 val;
+	const struct meson_uart_data *private_data = port->private_data;
+	u32 val = 0;
 
 	while (!meson_uart_tx_empty(port))
 		cpu_relax();
 
 	if (port->uartclk == 24000000) {
-		val = DIV_ROUND_CLOSEST(port->uartclk / 3, baud) - 1;
+		unsigned int xtal_div = 3;
+
+		if (private_data && private_data->has_xtal_div2) {
+			xtal_div = 2;
+			val |= AML_UART_BAUD_XTAL_DIV2;
+		}
+		val |= DIV_ROUND_CLOSEST(port->uartclk / xtal_div, baud) - 1;
 		val |= AML_UART_BAUD_XTAL;
 	} else {
 		val =  DIV_ROUND_CLOSEST(port->uartclk / 4, baud) - 1;
@@ -749,6 +761,7 @@ static int meson_uart_probe(struct platform_device *pdev)
 	port->x_char = 0;
 	port->ops = &meson_uart_ops;
 	port->fifosize = fifosize;
+	port->private_data = (void *)device_get_match_data(&pdev->dev);
 
 	meson_ports[pdev->id] = port;
 	platform_set_drvdata(pdev, port);
-- 
2.33.1