From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755191AbbGOKFQ (ORCPT ); Wed, 15 Jul 2015 06:05:16 -0400 Received: from eddie.linux-mips.org ([148.251.95.138]:59386 "EHLO cvs.linux-mips.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751938AbbGOKFL (ORCPT ); Wed, 15 Jul 2015 06:05:11 -0400 Date: Wed, 15 Jul 2015 12:05:03 +0200 From: Ralf Baechle To: Chris Packham Cc: linux-mips@linux-mips.org, Daniel Schwierzeck , "Steven J. Hill" , "Maciej W. Rozycki" , linux-kernel@vger.kernel.org, James Hogan , Markos Chandras , Paul Burton Subject: Re: [RFC PATCH v1] mips: Use unsigned int when reading CP0 registers Message-ID: <20150715100503.GA22385@linux-mips.org> References: <1436913870-17738-1-git-send-email-judge.packham@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1436913870-17738-1-git-send-email-judge.packham@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 15, 2015 at 10:44:30AM +1200, Chris Packham wrote: > Update __read_32bit_c0_register() and __read_32bit_c0_ctrl_register() to > use "unsigned int res;" instead of "int res;". There is little reason to > treat these register values as signed. They are either counters (which > by definition are unsigned) or are made up of various bit fields to be > interpreted as per the CPU datasheet. > > Signed-off-by: Chris Packham > > --- > This has come up via u-boot[1] which sync's asm/mipsregs.h with the > kernel. In u-boots case the value read from read_c0_count() is assigned > to an unsigned long [2] which triggers a sign extension and causes a > bug. > > U-boot should probably be more explicit about the types used for the > timer_read_counter() API but that aside is there any reason to treat > these values as signed integers? A quick grep around the arch/mips makes > me thing that there may be some bugs lurking when read_c0_count() starts > to yield a negative value but I haven't really explored any of them. Known issue but I've always been concerned about math with cycle values like: unsigned int now, timeout = read_c0_counter() + a_bit_of_time; waste_some_time(); if (timeout - read_c0_counter() < 0) do_timeout_stuff(); Which now with both variables being unsigned would yield a positive value thus the if would never be taken. This particular construction GCC would warn about but there are other, constructs that wouldn't trigger a warning. I don't even want to think about what C type propagation rules say about mixing signed and unsigned types. Whenever such knowledge is required to figure out what a piece of code is doing it probably should be considered broken anyway - but the mess resulting from unwanted sign is no better! Anyway, I've queued your patch for 4.3. Thanks! > I also notice that read_32bit_cp1_register has a similar issue. I > haven't touched it at this stage but it probably makes sense to do so > for consistency if the CP0 macros are changed. Looking at the users of > read_32bit_cp1_register() it's probably less of an issue. I've cooked up a patch for read_32bit_cp1_register and queued it for 4.3. Ralf