Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio
From: Jiri Slaby <hidden>
Date: 2012-09-28 13:03:06
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On 09/28/2012 02:19 PM, Alexey Brodkin wrote:
quoted hunk ↗ jump to hunk
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