Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754605Ab3ILEha (ORCPT ); Thu, 12 Sep 2013 00:37:30 -0400 Received: from mail-vc0-f174.google.com ([209.85.220.174]:36811 "EHLO mail-vc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752842Ab3ILEh2 (ORCPT ); Thu, 12 Sep 2013 00:37:28 -0400 MIME-Version: 1.0 In-Reply-To: References: <20130910190901.GA10105@radagast> <20130911183825.GC10105@radagast> <20130911190033.GD10105@radagast> <20130911204714.GH10105@radagast> From: Alexey Pelykh Date: Thu, 12 Sep 2013 07:37:07 +0300 Message-ID: Subject: Re: commit 5fe212364 causes division by zero with large bauds To: balbi@ti.com Cc: Tony Lindgren , Greg KH , Linux OMAP Mailing List , linux-serial@vger.kernel.org, Linux Kernel Mailing List Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8472 Lines: 209 Actually, here it is, but not formatted properly diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c index 816d1a2..146e712 100644 --- a/drivers/tty/serial/omap-serial.c +++ b/drivers/tty/serial/omap-serial.c @@ -240,14 +240,14 @@ serial_omap_baud_is_mode16(struct uart_port *port, unsigned int baud) { unsigned int n13 = port->uartclk / (13 * baud); unsigned int n16 = port->uartclk / (16 * baud); - int baudAbsDiff13 = baud - (port->uartclk / (13 * n13)); - int baudAbsDiff16 = baud - (port->uartclk / (16 * n16)); + int baudAbsDiff13 = n13 ? (baud - (port->uartclk / (13 * n13))) : INT_MAX; + int baudAbsDiff16 = n16 ? (baud - (port->uartclk / (16 * n16))) : INT_MAX; if(baudAbsDiff13 < 0) baudAbsDiff13 = -baudAbsDiff13; if(baudAbsDiff16 < 0) baudAbsDiff16 = -baudAbsDiff16; - return (baudAbsDiff13 > baudAbsDiff16); + return (baudAbsDiff13 >= baudAbsDiff16); } /* @@ -258,13 +258,13 @@ serial_omap_baud_is_mode16(struct uart_port *port, unsigned int baud) static unsigned int serial_omap_get_divisor(struct uart_port *port, unsigned int baud) { - unsigned int divisor; + unsigned int mode; if (!serial_omap_baud_is_mode16(port, baud)) - divisor = 13; + mode = 13; else - divisor = 16; - return port->uartclk/(baud * divisor); + mode = 16; + return port->uartclk/(mode * baud); } static void serial_omap_enable_ms(struct uart_port *port) On Thu, Sep 12, 2013 at 7:32 AM, Alexey Pelykh wrote: > On Wed, Sep 11, 2013 at 11:47 PM, Felipe Balbi wrote: >> Hi, >> >> On Wed, Sep 11, 2013 at 10:19:47PM +0300, Alexey Pelykh wrote: >>> On Wed, Sep 11, 2013 at 10:00 PM, Felipe Balbi wrote: >>> > On Wed, Sep 11, 2013 at 09:48:13PM +0300, Alexey Pelykh wrote: >>> >> On Wed, Sep 11, 2013 at 9:38 PM, Felipe Balbi wrote: >>> >> > Hi, >>> >> > >>> >> > On Wed, Sep 11, 2013 at 09:22:26AM +0300, Alexey Pelykh wrote: >>> >> >> Hi Felipe, >>> >> >> >>> >> >> Thanks for finding this issue. Indeed, there is a bug on 3M+ baud >>> >> >> rates. First patch is close to a complete fix, but still contains >>> >> >> div-by-zero issue. Here is my version: >>> >> >> >>> >> >> diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c >>> >> >> index 816d1a2..808a880 100644 >>> >> >> --- a/drivers/tty/serial/omap-serial.c >>> >> >> +++ b/drivers/tty/serial/omap-serial.c >>> >> >> @@ -240,8 +240,8 @@ serial_omap_baud_is_mode16(struct uart_port *port, >>> >> >> unsigned int baud) >>> >> >> { >>> >> >> unsigned int n13 = port->uartclk / (13 * baud); >>> >> >> unsigned int n16 = port->uartclk / (16 * baud); >>> >> >> - int baudAbsDiff13 = baud - (port->uartclk / (13 * n13)); >>> >> >> - int baudAbsDiff16 = baud - (port->uartclk / (16 * n16)); >>> >> >> + int baudAbsDiff13 = n13 ? (baud - (port->uartclk / (13 * n13))) : INT_MAX; >>> >> >> + int baudAbsDiff16 = n16 ? (baud - (port->uartclk / (16 * n16))) : INT_MAX; >>> >> > >>> >> > IOW: >>> >> > >>> >> > int baudAbsDiff13 = 0; >>> >> > >>> >> > if (n13) >>> >> > baudAbsDiff13 = (baud - (port->uartclk / (13 * n13))); >>> >> >>> >> Not quite same code, INT_MAX instead of 0. With 0 a div-by-zero >>> >> exception will still occur on 3686400. >>> > >>> > why, there's no division after that point, right ? Besides, >>> > serial_omap_baud_is_mode16() is supposed to return a boolean value. >>> > >>> > Setting baudAbsDiff1[36] to 0 will cause no problems, you're only using >>> > that value with a boolean expression, no divisions whatsoever. Division >>> > is done after using serial_omap_baud_is_mode16() to initialize divisor >>> > to 13 or 16 (which is a misnamer, since that's the oversampling >>> > parameter). >>> > >>> >>> Yes, variables are a bit misnamed, that should be fixed someday. >>> Regarding 0 vs INT_MAX, in case of 0 values will be >>> 300: divisor = 12307 (13) >>> 600: divisor = 6153 (13) >>> 1200: divisor = 3076 (13) >>> 2400: divisor = 1538 (13) >>> 4800: divisor = 625 (16) >>> 9600: divisor = 384 (13) >>> 14400: divisor = 256 (13) >>> 19200: divisor = 192 (13) >>> 28800: divisor = 128 (13) >>> 38400: divisor = 96 (13) >>> 57600: divisor = 64 (13) >>> 115200: divisor = 32 (13) >>> 230400: divisor = 16 (13) >>> 460800: divisor = 8 (13) >>> 921600: divisor = 4 (13) >>> 1000000: divisor = 3 (16) >>> 1843200: divisor = 2 (13) >>> 3000000: divisor = 1 (16) >>> 3686400: divisor = 0 (16) << error here, should be 1 (13), as it is with INT_MAX >> >> I get it now... your boolean check wants to use the closer baud to >> requested baud, so it's mode16 if the delta between baudAbsDiff16 and >> requested rate is less than delta between baudAbsDiff13 and requested >> baud. >> >>> >> > which is exactly what my patch did. I fail to see where division by zero >>> >> > would be coming from. >>> >> > >>> >> >> if(baudAbsDiff13 < 0) >>> >> >> baudAbsDiff13 = -baudAbsDiff13; >>> >> >> if(baudAbsDiff16 < 0) >>> >> >> >>> >> >> >>> >> >> With 48MHz UART clock, it will give >>> >> >> 300: divisor = 12307 (13), real rate 300 (0.000000%) >>> >> >> 600: divisor = 6153 (13), real rate 600 (0.000000%) >>> >> >> 1200: divisor = 3076 (13), real rate 1200 (0.000000%) >>> >> >> 2400: divisor = 1538 (13), real rate 2400 (0.000000%) >>> >> > >>> >> > TRM has these all set with oversampling of 16. In fact only 460800, >>> >> > 921600, 1843200 and 3686400 should be using oversampling of 13. >>> >> > >>> >> >>> >> That's true, but TRM anyways does not contain all possible baud rates >>> >> (1M e.g.). IMO, as long as error rate is the same as in TRM, >>> >> it makes no difference what combination of (mode, divisor) to use. >>> >> >>> >> > -- >>> >> > balbi >>> >> >>> >> A complex solution may be implemented: use LUT for baud rates that TRM >>> >> defines explicitly, and use calculation if lookup failed. >>> > >>> > why would you try calculating anything if there is nothing in the table >>> > which can support it ? Whatever is in the lookup table, are the only >>> > baud rates the SoC supports, right ? >>> > >>> >>> Actually, I haven't found any statement in TRM, which would mention >>> that listed baudrates in referenced table are the only supported baud >>> rates, >>> and all others are illegal. >> >> "The UART clocks are connected to produce a baud rate of up to 3.6 Mbps. >> Table 24-122 lists the *supported* baud rates, requested divisor, and >> corresponding error versus the standard baud rate." >> >>> At least 1M which I use extensively works perfectly, and I can not >>> figure out any idea why it would not do so. >> >> it might very well work, but it's not officially *supported* by the IP. > > That's true, but I don't see any reason why driver should disallow > usage of baud rates that are not supported, but possible by hardware: > "The UART clocks are connected to produce a baud rate of up to 3.6M bits/s." > >> >> -- >> balbi > > I've changed calculation a bit to give priority to mode16, and now it > gives TRM table as-is + extra baud rates > 300: divisor = 10000 (16), real rate 300 (0.000000%) > 600: divisor = 5000 (16), real rate 600 (0.000000%) > 1200: divisor = 2500 (16), real rate 1200 (0.000000%) > 2400: divisor = 1250 (16), real rate 2400 (0.000000%) > 4800: divisor = 625 (16), real rate 4800 (0.000000%) > 9600: divisor = 312 (16), real rate 9615 (0.156250%) > 14400: divisor = 208 (16), real rate 14423 (0.159722%) > 19200: divisor = 156 (16), real rate 19230 (0.156250%) > 28800: divisor = 104 (16), real rate 28846 (0.159722%) > 38400: divisor = 78 (16), real rate 38461 (0.158854%) > 57600: divisor = 52 (16), real rate 57692 (0.159722%) > 115200: divisor = 26 (16), real rate 115384 (0.159722%) > 230400: divisor = 13 (16), real rate 230769 (0.160156%) > 460800: divisor = 8 (13), real rate 461538 (0.160156%) > 921600: divisor = 4 (13), real rate 923076 (0.160156%) > 1000000: divisor = 3 (16), real rate 1000000 (0.000000%) > 1843200: divisor = 2 (13), real rate 1846153 (0.160211%) > 3000000: divisor = 1 (16), real rate 3000000 (0.000000%) > 3686400: divisor = 1 (13), real rate 3692307 (0.160238%) > > If that's acceptable behavior, I'll prepare a patch. > > Thanks, > Alexey -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/