linux-kernel - Re: [PATCH] tty/8250_early: Prevent rounding error in uartclk to baud ratio

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <50659FA8.7020200@suse.cz>
Date:	Fri, 28 Sep 2012 15:01:28 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
CC:	gregkh@...e.de, alan@...ux.intel.com, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, Vineet.Gupta1@...opsys.com
Subject: Re: [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@...opsys.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/