On Saturday 29 September 2012 12:37 PM, Jiri Slaby wrote: > On 09/29/2012 08:40 AM, Vineet Gupta wrote: >> On Friday 28 September 2012 06:32 PM, Jiri Slaby wrote: >>> On 09/28/2012 02:19 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@xxxxxxxxxxxx> >>>> --- >>>> 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? >> Actually the issue showed up when using the stock 8250 driver for >> Synopsys DW UART. This was on a FPGA with ~50MHz clk. When we enabled >> early serial, we saw garbage which Alexey narrowed down to the rounding >> error. So the bug had been latent and it only showed up with such low >> clk rates. > Yes, this is a perfect argument I was asking for and should have been a > part of the commit... First timer's anxiety I suppose ! Per your DIV_ROUND_CLOSEST() suggestion, Alexey has already sent out a v2 patch. I presume he needs to respin a v3 to beef up the commitlog along the lines of above ! Thx, -Vineet -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
