On Fri, 30 Jun 2023, Ruihong Luo wrote:
> 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")
Add also:
Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Ruihong Luo <colorsu1922@xxxxxxxxx>
> ---
> v3:
> - modify the commit message
> - use personal email to sign
> 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);
The change looks good but I'd prefer the variable to be called old_dlf
which is inline how 8250_port names its variables does when it preserves a
registers across some auto-detection write.
--
i.
