Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754162Ab3ILEdR (ORCPT ); Thu, 12 Sep 2013 00:33:17 -0400 Received: from mail-vc0-f181.google.com ([209.85.220.181]:37309 "EHLO mail-vc0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752661Ab3ILEdP (ORCPT ); Thu, 12 Sep 2013 00:33:15 -0400 MIME-Version: 1.0 In-Reply-To: <20130911204714.GH10105@radagast> References: <20130910190901.GA10105@radagast> <20130911183825.GC10105@radagast> <20130911190033.GD10105@radagast> <20130911204714.GH10105@radagast> From: Alexey Pelykh Date: Thu, 12 Sep 2013 07:32:54 +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: 6805 Lines: 163 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/