From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6DDB3C4361B for ; Wed, 9 Dec 2020 11:28:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00A0323A22 for ; Wed, 9 Dec 2020 11:28:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730660AbgLIL2E convert rfc822-to-8bit (ORCPT ); Wed, 9 Dec 2020 06:28:04 -0500 Received: from aposti.net ([89.234.176.197]:60676 "EHLO aposti.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727970AbgLIL2D (ORCPT ); Wed, 9 Dec 2020 06:28:03 -0500 Date: Wed, 09 Dec 2020 11:27:07 +0000 From: Paul Cercueil Subject: Re: [PATCH 1/2] if_enabled.h: Add IF_ENABLED_OR_ELSE() =?UTF-8?Q?and=0D=0A?= IF_ENABLED() macros To: Ard Biesheuvel Cc: Linus Walleij , Arnd Bergmann , MIPS , Linux Kernel Mailing List , od@zcrc.me, Linux ARM Message-Id: <75L2LQ.XOYPRHI9W3U9@crapouillou.net> In-Reply-To: References: <20201208164821.2686082-1-paul@crapouillou.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ard, Le mer. 9 déc. 2020 à 10:38, Ard Biesheuvel a écrit : > On Tue, 8 Dec 2020 at 17:49, Paul Cercueil > wrote: >> >> Introduce a new header , that brings two new >> macros: >> IF_ENABLED_OR_ELSE() and IF_ENABLED(). >> >> IF_ENABLED_OR_ELSE(CONFIG_FOO, a, b) evaluates to (a) if CONFIG_FOO >> is set >> to 'y' or 'm', (b) otherwise. It is used internally to define the >> IF_ENABLED() macro. The (a) and (b) arguments must be of the same >> type. >> >> IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set >> to 'y' >> or 'm', NULL otherwise. The (ptr) argument must be a pointer. >> >> The IF_ENABLED() macro can be very useful to help GCC drop dead >> code. >> >> For instance, consider the following: >> >> #ifdef CONFIG_FOO_SUSPEND >> static int foo_suspend(struct device *dev) >> { >> ... >> } >> #endif >> >> static struct pm_ops foo_ops = { >> #ifdef CONFIG_FOO_SUSPEND >> .suspend = foo_suspend, >> #endif >> }; >> >> While this works, the foo_suspend() macro is compiled conditionally, >> only when CONFIG_FOO_SUSPEND is set. This is problematic, as there >> could >> be a build bug in this function, we wouldn't have a way to know >> unless >> the config option is set. >> >> An alternative is to declare foo_suspend() always, but mark it as >> maybe >> unused: >> >> static int __maybe_unused foo_suspend(struct device *dev) >> { >> ... >> } >> >> static struct pm_ops foo_ops = { >> #ifdef CONFIG_FOO_SUSPEND >> .suspend = foo_suspend, >> #endif >> }; >> >> Again, this works, but the __maybe_unused attribute is required to >> instruct the compiler that the function may not be referenced >> anywhere, >> and is safe to remove without making a fuss about it. This makes the >> programmer responsible for tagging the functions that can be >> garbage-collected. >> >> With this patch, it is now possible to write the following: >> >> static int foo_suspend(struct device *dev) >> { >> ... >> } >> >> static struct pm_ops foo_ops = { >> .suspend = IF_ENABLED(CONFIG_FOO_SUSPEND, foo_suspend), >> }; >> >> The foo_suspend() function will now be automatically dropped by the >> compiler, and it does not require any specific attribute. >> >> Signed-off-by: Paul Cercueil > > Hi Paul, > > I understand the issue you are trying to address here, but please note > that there are many cases where the struct member in question will not > even be declared if the associated CONFIG option is not set, in which > case this will cause a compile error. Of course. This is for the case where the field is always present. For instance, (struct device_driver *)->pm is always present, independently of CONFIG_PM. Cheers, -Paul > >> --- >> include/linux/if_enabled.h | 22 ++++++++++++++++++++++ >> 1 file changed, 22 insertions(+) >> create mode 100644 include/linux/if_enabled.h >> >> diff --git a/include/linux/if_enabled.h b/include/linux/if_enabled.h >> new file mode 100644 >> index 000000000000..8189d1402340 >> --- /dev/null >> +++ b/include/linux/if_enabled.h >> @@ -0,0 +1,22 @@ >> +/* SPDX-License-Identifier: GPL-2.0 */ >> +#ifndef __LINUX_IF_ENABLED_H >> +#define __LINUX_IF_ENABLED_H >> + >> +#include >> +#include >> +#include >> + >> +/* >> + * IF_ENABLED_OR_ELSE(CONFIG_FOO, a, b) evaluates to (a) if >> CONFIG_FOO is set >> + * to 'y' or 'm', (b) otherwise. >> + */ >> +#define IF_ENABLED_OR_ELSE(option, a, b) \ >> + (BUILD_BUG_ON_ZERO(__same_type((a), (b))) || >> IS_ENABLED(option) ? (a) : (b)) >> + >> +/* >> + * IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is >> set to 'y' >> + * or 'm', NULL otherwise. >> + */ >> +#define IF_ENABLED(option, ptr) IF_ENABLED_OR_ELSE(option, ptr, >> NULL) >> + >> +#endif /* __LINUX_IF_ENABLED_H */ >> -- >> 2.29.2 >> >> >> _______________________________________________ >> linux-arm-kernel mailing list >> linux-arm-kernel@lists.infradead.org >> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel