serial: 8250_dw: Preserve original value of DLF register

Preserve the original value of the Divisor Latch Fraction (DLF) 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")
Cc: stable <stable@kernel.org>
Signed-off-by: Ruihong Luo <colorsu1922@gmail.com>
Link: https://lore.kernel.org/stable/20230713004235.35904-1-colorsu1922%40gmail.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230713004235.35904-1-colorsu1922@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 75f32f054ebb15e64b8fedc80bb743c431f3375f..84843e204a5e80e872ab840e9069cc73a018d38d 100644 (file)
--- 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, old_dlf;
 
        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  */
+       old_dlf = 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, old_dlf);
 
        if (reg) {
                pd->dlf_size = fls(reg);