From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753145Ab1GHGnx (ORCPT ); Fri, 8 Jul 2011 02:43:53 -0400 Received: from mail-qw0-f46.google.com ([209.85.216.46]:43268 "EHLO mail-qw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752278Ab1GHGnw convert rfc822-to-8bit (ORCPT ); Fri, 8 Jul 2011 02:43:52 -0400 MIME-Version: 1.0 In-Reply-To: <20110707233335.GA15120@kroah.com> References: <1309994990-4729-1-git-send-email-sergiu@chromium.org> <1309994990-4729-4-git-send-email-sergiu@chromium.org> <20110707130130.8dd02f01.akpm@linux-foundation.org> <20110707155426.fd95445f.akpm@linux-foundation.org> <20110707162727.f361f053.akpm@linux-foundation.org> <20110707233335.GA15120@kroah.com> Date: Fri, 8 Jul 2011 08:43:50 +0200 Message-ID: Subject: Re: [PATCH v3 3/3] char drivers: ramoops debugfs entry From: Marco Stornelli To: Greg KH Cc: Andrew Morton , Sergiu Iordache , "Ahmed S. Darwish" , Artem Bityutskiy , Kyungmin Park , linux-kernel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2011/7/8 Greg KH : > On Thu, Jul 07, 2011 at 04:27:27PM -0700, Andrew Morton wrote: >> On Thu, 7 Jul 2011 16:16:43 -0700 >> Sergiu Iordache wrote: >> >> > Ramoops currently dumps the log of a panic/oops in a memory area which >> > is known not to be overwritten on restart (for example 1MB starting at >> > 15MB). The way it works is by dividing the memory area in records of a >> > set size (fixed at 4K before my patches, configurable after) and by >> > dumping a record there for each oops/panic. The problem is that right >> > now you have to access that memory area through other means, such as >> > /dev/mem, which is not always possible. >> > >> > What my patch did was to add a debugfs entry which returns a valid >> > record each time (a single dump done by ramoops). The first call >> > returns the first dump. The first call after the last valid dump >> > returns an empty buffer. . >> >> Please fully describe this "record" in the v2 patch changelog.  We'll >> want to review it for endianness, 32/64-bit compat issues, >> maintainability, extensibility, etc. >> >> > After it has returned nothing, the next >> > calls return records from the start again. >> >> That sounds a bit weird.  One would expect it to keep returning zero, >> requiring userspace to lseek or close/open. >> >> > The validity of a dump is >> > checked by looking after the header. Any comments on this approach are >> > welcome. >> > >> > Changing the entry from debugfs to sysfs wouldn't be a problem. If >> > sysfs is a valid solution I'll come with a patch that updates the >> > documentation as well along with the sysfs entry. >> >> sysfs sounds OK to me.  Then again, sysfs is supposed to be >> one-value-per-file, so using it would be naughty. >> >> I dunno, I'd be inclined to abuse the sysfs rule and hope that nobody >> notices rather than create a fake char device.  But there's certainly >> plenty of precedent for the fake char driver. > > No, please don't abuse sysfs that way. > > Use debugfs or a char device node. > > thanks, > > greg k-h > I agree with Greg. I asked to not break the existent way to read data via /dev/mem because for me it's the right way to do this thing. However to do an easy *debug* a debugfs entry can be useful. IMHO, a "production" script/application that use debugfs instead of /dev/mem in this case is simply broken because the debugfs can't be like a system call or other kernel interaction mechanism. Debugfs should be used only for debug. Marco