Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751366AbaK1JKU (ORCPT ); Fri, 28 Nov 2014 04:10:20 -0500 Received: from lvps176-28-13-145.dedicated.hosteurope.de ([176.28.13.145]:38232 "EHLO lvps176-28-13-145.dedicated.hosteurope.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750938AbaK1JKJ (ORCPT ); Fri, 28 Nov 2014 04:10:09 -0500 From: Tim Sander To: Daniel Thompson Cc: Russell King - ARM Linux , Thomas Gleixner , Jason Cooper , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, patches@linaro.org, linaro-kernel@lists.linaro.org, John Stultz , Sumit Semwal , Dirk Behme , Daniel Drake , Dmitry Pervushin Subject: Re: [PATCH 3.18-rc3 v9 5/5] arm: smp: Handle ipi_cpu_backtrace() using FIQ (if available) Date: Fri, 28 Nov 2014 10:10:04 +0100 Message-ID: <13373554.4deCsZOMXS@dabox> Organization: Sander and Lightning User-Agent: KMail/4.14.2 (Linux/3.16.3; KDE/4.14.2; x86_64; ; ) In-Reply-To: <5475FD02.3030501@linaro.org> References: <1415968543-29469-1-git-send-email-daniel.thompson@linaro.org> <20141126131210.GI3836@n2100.arm.linux.org.uk> <5475FD02.3030501@linaro.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Daniel, Russell Am Mittwoch, 26. November 2014, 16:17:06 schrieb Daniel Thompson: > On 26/11/14 13:12, Russell King - ARM Linux wrote: > > On Wed, Nov 26, 2014 at 01:46:52PM +0100, Tim Sander wrote: > >> Hi Daniel > >> > >> Am Dienstag, 25. November 2014, 17:26:41 schrieb Daniel Thompson: > >>> Previous changes have introduced both a replacement default FIQ handler > >>> and an implementation of arch_trigger_all_cpu_backtrace for ARM but > >>> these are currently independent of each other. > >>> > >>> This patch plumbs together these features making it possible, on > >>> platforms > >>> that support it, to trigger backtrace using FIQ. > >> > >> Does this ipi handler interfere in any way with set_fiq_handler? > >> > >> As far as i know there is only one FIQ handler vector so i guess there is > >> a > >> potential conflict. But i have not worked with IPI's so i might be > >> completley wrong. > > > > First, the code in arch/arm/kernel/fiq.c should work with this new FIQ > > code in that the new FIQ code is used as the "default" handler (as > > opposed to the original handler which was a total no-op.) > > > > Secondly, use of arch/arm/kernel/fiq.c in a SMP system is really not a > > good idea: the FIQ registers are private to each CPU in the system, and > > there is no infrastructure to allow fiq.c to ensure that it loads the > > right CPU with the register information for the provided handler. Well given the races in the GIC v1. i have seen in the chips on my desk initializing with for_each_possible_cpu(cpu) work_on_cpu(cpu,..) is rather easy. > > So, use of arch/arm/kernel/fiq.c and the IPI's use of FIQ /should/ be > > mutually exclusive. Yes but i digress on the assessment that this a decision between SMP and non- SMP usage or the availbility of the GIC. > Agree with the above. Just to add... > > I am currently working to get NMI features from x86 land running on top > of the new default FIQ handler: arch_trigger_all_cpu_backtrace (with > Russell's patch), perf, hard lockup detector, kgdb. > > However I don't think anything I'm doing makes it very much harder than > it already is to use arch/arm/kernel/fiq.c . That said, other then > setting the GIC up nicely, I am not doing anything to make it easier either. > > I'd like to end up somewhere where if you want the NMI features (and > have a suitable device) you just use the default handler and it all just > works. If you need *Fast* Interrupt reQuests, proper old school "I want > to write an overclocked I2C slave in software" craziness and you can > pass on the improved debug features then set_fiq_handler() is still > there and still need extremely careful handling. Well i am not against these features as they assumably improve the backtrace, but it would be nice to have a config option which switches between set_fiq_handler usage and the other conflicting usages of the fiq. > The only thing I might have done to make your life worse is not provide > the code to dynamically shunt all the debug and performance monitoring > features back to group 1. All except the hard lockup detector will have > logic to fall back statically. This means making it dynamic shouldn't be > that hard. However since there is no code in the upstream kernel that > would use the code I don't plan to go there myself. I don't think this needs to by dynamic, but from a user perspective a config option would be really nice. Tim -- 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/