[PATCH] lib: utils/serial: Round UART8250 baud rate divisor to nearest integer

[Jessica Clarke]

On 21 Jan 2022, at 15:06, Jakub Luzny <jakub.luzny at codasip.com> wrote:
> 
> Previously, it was rounded down and that gives suboptimal results when
> non-standard clock sources or baud rates are used.
> 
> Signed-off-by: Jakub Luzny <jakub.luzny at codasip.com>
> ---
> lib/utils/serial/uart8250.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/utils/serial/uart8250.c b/lib/utils/serial/uart8250.c
> index 1cf6624..ccc7aa0 100644
> --- a/lib/utils/serial/uart8250.c
> +++ b/lib/utils/serial/uart8250.c
> @@ -101,7 +101,7 @@ int uart8250_init(unsigned long base, u32 in_freq, u32 baudrate, u32 reg_shift,
> 	uart8250_in_freq   = in_freq;
> 	uart8250_baudrate  = baudrate;
> 
> -	bdiv = uart8250_in_freq / (16 * uart8250_baudrate);
> +	bdiv = (uart8250_in_freq + 8 * uart8250_baudrate) / (16 * uart8250_baudrate);

This could do weird things if the numbers are sufficiently large to
overflow (unlikely but maybe possible). You can instead use the simpler:

  (uart8250_in_freq / (8 * uart8250_baudrate) + 1) / 2

which is approximately what FreeBSD does (it shifts rather than
multiplies and divides).

Jess