[PATCH RFC] tty: serial: meson: add new compatible for gxl uart

[Heiner Kallweit]

My SC2 test system (ah212-based HK1 RBOX X4) uses an uboot console
speed of 921600 bps. Being able to use the same speed in Linux
requires using a 12MHz clock source. Ideally we'd set the new
compatible to the first chip version supporting the
AML_UART_BAUD_XTAL_DIV2 bit. Not having access to chip documentation
I'd appreciate a hint on when this bit was introduced.
If fine with you in general the final series would include a
separate patch for the new compatible.

Note: This patch doesn't consider "tty: serial: meson:
Use DIV_ROUND_CLOSEST to calculate baud rates" yet and therefore
won't apply cleanly on linux-next.
I locally used the same DIV_ROUND_CLOSEST() changes before.
----------------------------------------------------------------
Newer SoC versions support using XTAL / 2 (12MHz) as clock source
for the UART. This clock source allows to support 921600 bps as used
by vendor uboot on certain systems like ah212 ref board.

12MHz / 13 = 923076 bps
With the currently used 8MHz clock source only 888888bps or
1000000 bps are possible.

Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
---
 drivers/tty/serial/meson_uart.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
index 2bf1c57e0..0a0a0636f 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
@@ -299,10 +300,15 @@ static void meson_uart_change_speed(struct uart_port *port, unsigned long baud)
 		cpu_relax();
 
 	if (port->uartclk == 24000000) {
-		val = ((port->uartclk / 3) / baud) - 1;
-		val |= AML_UART_BAUD_XTAL;
+		if (of_device_is_compatible(port->dev->of_node, "amlogic,meson-gxl-uart")) {
+			val = DIV_ROUND_CLOSEST(port->uartclk, 2 * baud) - 1;
+			val |= AML_UART_BAUD_XTAL | AML_UART_BAUD_XTAL_DIV2;
+		} else {
+			val = DIV_ROUND_CLOSEST(port->uartclk, 3 * baud) - 1;
+			val |= AML_UART_BAUD_XTAL;
+		}
 	} else {
-		val = ((port->uartclk * 10 / (baud * 4) + 5) / 10) - 1;
+		val = DIV_ROUND_CLOSEST(port->uartclk, 4 * baud) - 1;
 	}
 	val |= AML_UART_BAUD_USE;
 	writel(val, port->membase + AML_UART_REG5);
@@ -782,6 +788,7 @@ static const struct of_device_id meson_uart_dt_match[] = {
 	{ .compatible = "amlogic,meson8-uart" },
 	{ .compatible = "amlogic,meson8b-uart" },
 	{ .compatible = "amlogic,meson-gx-uart" },
+	{ .compatible = "amlogic,meson-gxl-uart" },
 	{ /* sentinel */ },
 };
 MODULE_DEVICE_TABLE(of, meson_uart_dt_match);
-- 
2.35.3