From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mikael Pettersson Subject: Re: gcc 4.8.3 miscompiles drivers/net/ethernet/freescale/fec_main.c ?! Date: Wed, 10 Sep 2014 14:32:20 +0200 Message-ID: <21520.17620.519242.597681@gargle.gargle.HOWL> References: <20140910110943.3673dbb4@archvile> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: Russell King , "David S. Miller" , netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org To: David Jander Return-path: Received: from mail-la0-f50.google.com ([209.85.215.50]:43509 "EHLO mail-la0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750899AbaIJMc0 (ORCPT ); Wed, 10 Sep 2014 08:32:26 -0400 Received: by mail-la0-f50.google.com with SMTP id ty20so6328380lab.9 for ; Wed, 10 Sep 2014 05:32:25 -0700 (PDT) In-Reply-To: <20140910110943.3673dbb4@archvile> Sender: netdev-owner@vger.kernel.org List-ID: David Jander writes: > > Hi, > > I am seeing a strange problem when building a recent kernel with gcc-4.8.3 for > armv7-a that contains the following patch: > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit?id=bfd4ecdd87d350e19457fe0d02fa1e046774c44e > > Unfortunately I am not good enough at reading ARM assembly output from GCC to > understand whats going wrong, so I am asking for help. > > I started noticing ethernet packet loss on a i.MX6 board after upgrading the > kernel from 3.16-rc-something to latest mainline. The problem is very easy to > reproduce so I started git-bisecting. Git bisect gave me the above patch as > the culprit, and indeed: Without the patch a flood-ping goes fine (just one > dot on screen, no lost packets). I apply the patch and the dots start filling > the screen instantly. > > I am compiling the kernel using Pengutronix's OSELAS toolchain version > 2013.12.1, which is based on linaro gcc-4.8.3 without any relevant patches > AFAIK. Linaro's toolchain is itself heavily modified compared to FSF gcc-4.8.3, so first please try a pure vanilla FSF gcc-4.8.3, and then a likewise vanilla gcc-4.9.1. If those also cause the malfunction, then you have proof for a bug in upstream gcc (or possibly undefined code in the kernel), otherwise the bug is likely Linaro's. > Compiling with -O2 breaks the code, while -Os seems to produce a correctly > working kernel. > > I decided to make changes to the code and see if I could find other ways to > "fix" the problem, and I got the following result: > > The above mentioned patch introduces the static function fec_enet_hwtstamp() > near line 1068 of fec_main.c. If I make an exact copy of this function, where > I only change the name (e.g. fec_enet_hwtstamp2), and change one of the two > places this function is called to instead use the other name, GCC inlines both > copies and the problem disappears! > > Since I am not very good at GCC internals nor do I know this piece of code in > fec_main.c very well, I am asking here for help in hunting down the real bug, > which I suspect is in GCC... but I want to know for sure. > > Best regards, > > -- > David Jander > Protonic Holland. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel --