serial: 8250_dw: Preserve original value of DLF register

commit 748c5ea8b8796ae8ee80b8d3a3d940570b588d59 upstream.

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 6d6a78eead3efcf256be4283a6a8945ff21d8879..1cf229cca59281923fd78199889ec5369dc46e9f 100644 (file)
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -80,7 +80,7 @@ static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
 void dw8250_setup_port(struct uart_port *p)
 {
        struct uart_8250_port *up = up_to_u8250p(p);
-       u32 reg;
+       u32 reg, old_dlf;
 
        /*
         * If the Component Version Register returns zero, we know that
@@ -93,9 +93,11 @@ 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) {
                struct dw8250_port_data *d = p->private_data;