Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754954AbbLKXa7 (ORCPT ); Fri, 11 Dec 2015 18:30:59 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:58168 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754040AbbLKXa4 (ORCPT ); Fri, 11 Dec 2015 18:30:56 -0500 Date: Fri, 11 Dec 2015 15:30:54 -0800 From: Andrew Morton To: Russell King - ARM Linux Cc: Petr Mladek , Geert Uytterhoeven , Peter Zijlstra , Steven Rostedt , Daniel Thompson , Jiri Kosina , Ingo Molnar , Thomas Gleixner , "linux-kernel@vger.kernel.org" , the arch/x86 maintainers , "linux-arm-kernel@lists.infradead.org" , "adi-buildroot-devel@lists.sourceforge.net" , Cris , Linux MIPS Mailing List , "linuxppc-dev@lists.ozlabs.org" , linux-s390 , Linux-sh list , sparclinux Subject: Re: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable Message-Id: <20151211153054.48da5d4139b93dd4ed438f4c@linux-foundation.org> In-Reply-To: <20151211232113.GZ8644@n2100.arm.linux.org.uk> References: <1449667265-17525-1-git-send-email-pmladek@suse.com> <1449667265-17525-5-git-send-email-pmladek@suse.com> <20151211124159.GB3729@pathway.suse.cz> <20151211145725.b0e81bb4bb18fcd72ef5f557@linux-foundation.org> <20151211232113.GZ8644@n2100.arm.linux.org.uk> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2159 Lines: 46 On Fri, 11 Dec 2015 23:21:13 +0000 Russell King - ARM Linux wrote: > On Fri, Dec 11, 2015 at 02:57:25PM -0800, Andrew Morton wrote: > > This is a bit messy. NEED_PRINTK_NMI is an added-on hack for one > > particular arm variant. From the changelog: > > > > "One exception is arm where the deferred printing is used for > > printing backtraces even without NMI. For this purpose, we define > > NEED_PRINTK_NMI Kconfig flag. The alternative printk_func is > > explicitly set when IPI_CPU_BACKTRACE is handled." > > > > > > - why does arm needs deferred printing for backtraces? > > > > - why is this specific to CONFIG_CPU_V7M? > > > > - can this Kconfig logic be cleaned up a bit? > > I think this comes purely from this attempt to apply another round of > cleanups to the nmi backtrace work I did. > > As I explained when I did that work, the vast majority of ARM platforms > are unable to trigger anything like a NMI - the FIQ is something that's > generally a property of the secure monitor, and is not accessible to > Linux. However, there are platforms where it is accessible. OK, thanks. So "not needed at present, might be needed in the future, useful for out-of-tree debug code"? > I'm personally happy with the existing code, and I've been wondering why > there's this effort to apply further cleanups - to me, the changelogs > don't seem to make that much sense, unless we want to start using > printk() extensively in NMI functions - using the generic nmi backtrace > code surely gets us something that works across all architectures... Yes, I was scratching my head over that. The patchset takes an nmi-safe all-cpu-backtrace and generalises that into an nmi-safe printk. That *sounds* like a good thing to do but yes, some additional justification would be helpful. What real-world value does this patchset really bring to real-world users? -- 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/