[PATCH v2] serial: 8250: Preserve original value of DLF register

[Ruihong Luo <colorsu1922@gmail.com>]

This commit is aimed at preserving the original value of the
DLF(Divisor Latch Fraction Register). When the DLF register is
modified without preservation, it can disrupt the baudrate settings
established by firmware or bootloader , leading to data corruption
and the generation of unreadable or distorted characters.

Fixes: 701c5e73b296 ("serial: 8250_dw: add fractional divisor support")
Signed-off-by: Ruihong Luo <luoruihong@xiaomi.com>
---
v2:
- add a tag
 drivers/tty/serial/8250/8250_dwlib.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 75f32f054ebb..d30957722da8 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -244,7 +244,7 @@ void dw8250_setup_port(struct uart_port *p)
 	struct dw8250_port_data *pd = p->private_data;
 	struct dw8250_data *data = to_dw8250_data(pd);
 	struct uart_8250_port *up = up_to_u8250p(p);
-	u32 reg;
+	u32 reg, orig;
 
 	pd->hw_rs485_support = dw8250_detect_rs485_hw(p);
 	if (pd->hw_rs485_support) {
@@ -270,9 +270,11 @@ void dw8250_setup_port(struct uart_port *p)
 	dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
 		(reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);
 
+	/* Preserve value written by firmware or bootloader  */
+	orig = dw8250_readl_ext(p, DW_UART_DLF);
 	dw8250_writel_ext(p, DW_UART_DLF, ~0U);
 	reg = dw8250_readl_ext(p, DW_UART_DLF);
-	dw8250_writel_ext(p, DW_UART_DLF, 0);
+	dw8250_writel_ext(p, DW_UART_DLF, orig);
 
 	if (reg) {
 		pd->dlf_size = fls(reg);
-- 
2.39.2