From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755945Ab1ATXgO (ORCPT ); Thu, 20 Jan 2011 18:36:14 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:45982 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754115Ab1ATXgN (ORCPT ); Thu, 20 Jan 2011 18:36:13 -0500 Date: Thu, 20 Jan 2011 15:35:28 -0800 From: Andrew Morton To: Mark Brown Cc: Kim Kyuwon , Kim Kyuwon , Richard Purdie , linux-kernel@vger.kernel.org Subject: Re: [PATCH] leds: Convert bd2802 driver to dev_pm_ops Message-Id: <20110120153528.4a535065.akpm@linux-foundation.org> In-Reply-To: <20110120232409.GA23862@opensource.wolfsonmicro.com> References: <1295559395-28942-1-git-send-email-broonie@opensource.wolfsonmicro.com> <20110120151201.8dca7a7e.akpm@linux-foundation.org> <20110120232409.GA23862@opensource.wolfsonmicro.com> X-Mailer: Sylpheed 3.0.2 (GTK+ 2.20.1; 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 On Thu, 20 Jan 2011 23:24:09 +0000 Mark Brown wrote: > On Thu, Jan 20, 2011 at 03:12:01PM -0800, Andrew Morton wrote: > > > CONFIG_PM=n: > > To be honest I've been forming the opinion that this is just cruft these > days - it's hard to see a modern Linux system where you're sufficiently > space constrained to want to turn it off without also being sufficiently > power constrained to want to turn it on and it's hassle to maintain it. Could be. It's hard to think of a machine which is small enough to care about ~10k of kernel text but which doesn't care about power consumption. > That said... > > > It would be nice to fix all this via automagic within the > > SIMPLE_DEV_PM_OPS() implementation but I can't see a way of doing that :( > > ...the problem here is that the macro is doing roughly the right magic > but the original driver wasn't ifdefing the suspend and resume stuff at > all. If the were only defining the suspend and resume functions under > CONFIG_PM_SLEEP it should build cleanly. Since the original driver > didn't have the ifdefs I didn't add or update them. > > This means the pm_ops can be unconditionally defined which seems to be > the preferred idiom for this stuff. If SIMPLE_DEV_PM_OPS() didn't do > the stuff it's doing then the warnings would vanish in the same way they > did originally, by virtue of the functions being unconditionally > referenced from the vtable. It's tricky to get the compiler/linker to discard code and data without triggering an unused-var warning. cpu_notifier() does it by emitting a reference: static void foo(..) { ... } bar() { ... do { (void)(foo); } while (0) ... } but this is only possible because cpu_notifier() is "called" within a function. Whereas SIMPLE_DEV_PM_OPS() is a static initialisation thingy. We could emit a dummy function but then we get a warning about the unused dummy function. Can the dummy function refer to itself and thus squish the warning? akpm:/home/akpm> cat t.c static void foo(void) { do { (void)(foo); } while (0); } akpm:/home/akpm> gcc -c -Wall t.c t.c:1: warning: 'foo' defined but not used Nope. This works. lol. akpm:/home/akpm> cat t.c static void bar(void); static void foo(void) { do { (void)(bar); } while (0); } static void bar(void) { do { (void)(foo); } while (0); } akpm:/home/akpm> gcc -O2 -c -Wall t.c akpm:/home/akpm> size t.o text data bss dec hex filename 0 0 0 0 0 t.o