From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757791Ab1GLGlx (ORCPT ); Tue, 12 Jul 2011 02:41:53 -0400 Received: from mail-qw0-f46.google.com ([209.85.216.46]:46613 "EHLO mail-qw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754667Ab1GLGlw convert rfc822-to-8bit (ORCPT ); Tue, 12 Jul 2011 02:41:52 -0400 MIME-Version: 1.0 In-Reply-To: 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: Tue, 12 Jul 2011 08:41:51 +0200 Message-ID: Subject: Re: [PATCH v3 3/3] char drivers: ramoops debugfs entry From: Marco Stornelli To: Sergiu Iordache Cc: Greg KH , Andrew Morton , "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 2011/7/11 Sergiu Iordache : > On Thu, Jul 7, 2011 at 11:43 PM, Marco Stornelli > wrote: >> 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 > > Any consensus/decision on how to go on with this patch idea? > > The options that I see right now are: > - keep access through /dev/mem only (but access to /dev/mem is > sometimes restricted); > - keep the debugfs entry as well(as in the patch); > - remove the debugfs entry and add a char driver to access the memory > using read and seek operations. > > + the rejected(?) options from before > > Sergiu > For me the best option it's to use a sysfs/proc entry to export (read-only) the memory address, record size etc. At that point we can use a generic script/program to access via /dev/mem. However I let Andrew/Greg say the last word. Marco