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SubjectRe: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio
On 09/28/2012 02:03 PM, Alexey Brodkin wrote:
> Modify divisor to select the nearest baud rate divider rather than the
> lowest. It minimizes baud rate errors especially on low UART clock
> frequencies.
>
> For example, if uartclk is 33000000 and baud is 115200 the ratio is
> about 17.9 The current code selects 17 (5% error) but should select 18
> (0.5% error).
>
> On the same lines as following:
> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.9-rc3/2.6.9-rc3-mm2/broken-out/serial-pick-nearest-baud-rate-divider.patch
>
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> ---
> drivers/tty/serial/8250_early.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/8250_early.c b/drivers/tty/serial/8250_early.c
> index eaafb98..cfc46b7 100644
> --- a/drivers/tty/serial/8250_early.c
> +++ b/drivers/tty/serial/8250_early.c
> @@ -140,7 +140,7 @@ static void __init init_port(struct early_serial8250_device *device)
> serial_out(port, UART_FCR, 0); /* no fifo */
> serial_out(port, UART_MCR, 0x3); /* DTR + RTS */
>
> - divisor = port->uartclk / (16 * device->baud);
> + divisor = (port->uartclk + (8 * device->baud)) / (16 * device->baud);

So this should be in fact DIV_ROUND_CLOSEST(), right?

But anyway I'm missing any explanation of *why* this is needed? This
should be part of the commit log. Is there any bug you are fixing here?

thanks,
--
js
suse labs


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