From patchwork Mon Nov 23 04:54:02 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Masahiro Yamada X-Patchwork-Id: 11924375 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=-22.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham 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 14479C64E7D for ; Mon, 23 Nov 2020 04:55:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DBA04212CC for ; Mon, 23 Nov 2020 04:55:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nifty.com header.i=@nifty.com header.b="a57e5Kt9" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727813AbgKWEza (ORCPT ); Sun, 22 Nov 2020 23:55:30 -0500 Received: from conuserg-08.nifty.com ([210.131.2.75]:27327 "EHLO conuserg-08.nifty.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726958AbgKWEzV (ORCPT ); Sun, 22 Nov 2020 23:55:21 -0500 Received: from grover.flets-west.jp (softbank126090211135.bbtec.net [126.90.211.135]) (authenticated) by conuserg-08.nifty.com with ESMTP id 0AN4sGHi016676; Mon, 23 Nov 2020 13:54:20 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conuserg-08.nifty.com 0AN4sGHi016676 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1606107261; bh=8NgvnHhJy/QixGNY6u3RlD/wM4+7Ln+DlMo5AkRm9tw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=a57e5Kt9YcqObZXzoElxuvSmeViii5TgEmGhybHx79QvV6CG/4VNpc6XbNgWftNPP h7NyUttrr/JXsKIqR2jX5TBQGFeZF17wK4ZPjRy5UBtYgdMw7lWbYZqEXSYSM5gRZ5 pb3zCFZEovAOJtAdNvLXHcx79zzf13C9+hTHdeX3U9QuhWJNSpW70eOq52vjKrQaZ0 5Jdg8rNOF7kKdKcuWHo/A7Cn7E99sRsiPImj+m4KmfLb4V68SmKvl+JvSGTdS0Eqdd Ku7xdtj6MCXJAS8gQZvIASyr1gOY3EmIDON2SDQraUXAKP1f1ayahONtnNIUzMwjFo xMDU/B0qHKuaQ== X-Nifty-SrcIP: [126.90.211.135] From: Masahiro Yamada To: linux-kbuild@vger.kernel.org Cc: Randy Dunlap , Masahiro Yamada , Jonathan Corbet , Michal Marek , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 6/7] kbuild: doc: clarify the difference between extra-y and always-y Date: Mon, 23 Nov 2020 13:54:02 +0900 Message-Id: <20201123045403.63402-6-masahiroy@kernel.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20201123045403.63402-1-masahiroy@kernel.org> References: <20201123045403.63402-1-masahiroy@kernel.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org The difference between extra-y and always-y is obscure. Basically, Kbuild builds targets listed in extra-y and always-y in visited Makefiles without relying on any dependency. The difference is that extra-y is used to list the targets needed for vmlinux whereas always-y is used to list the targets that must be always built irrespective of final targets. Kbuild skips extra-y when it is building only modules (i.e. 'make modules'). This is the long-standing behavior since extra-y was introduced in 2003, and it is explained in that commit log [1]. For clarification, this is the extra-y vs always-y table: extra-y always-y 'make' o o 'make vmlinux' o o 'make modules' x o Kbuild skips extra-y also when building external modules since obviously it never builds vmlinux. Unfortunately, extra-y is wrongly used in many places of upstream code, and even in external modules. Using extra-y in external module Makefiles is wrong. What you should use is probably always-y or 'targets'. The current documentation for extra-y is misleading. I rewrote it, and moved it to the section 3.7. always-y is not documented anywhere. I added. [1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=f94e5fd7e5d09a56a60670a9bb211a791654bba8 Signed-off-by: Masahiro Yamada Reviewed-by: Randy Dunlap --- Documentation/kbuild/makefiles.rst | 110 +++++++++++++++++++---------- 1 file changed, 71 insertions(+), 39 deletions(-) diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst index 49afcb1d3695..159e470f2616 100644 --- a/Documentation/kbuild/makefiles.rst +++ b/Documentation/kbuild/makefiles.rst @@ -15,13 +15,15 @@ This document describes the Linux kernel Makefiles. --- 3.4 Objects which export symbols --- 3.5 Library file goals - lib-y --- 3.6 Descending down in directories - --- 3.7 Compilation flags - --- 3.8 Dependency tracking - --- 3.9 Custom Rules - --- 3.10 Command change detection - --- 3.11 $(CC) support functions - --- 3.12 $(LD) support functions - --- 3.13 Script Invocation + --- 3.7 Non-builtin vmlinux targets - extra-y + --- 3.8 Always built goals - always- + --- 3.9 Compilation flags + --- 3.10 Dependency tracking + --- 3.11 Custom Rules + --- 3.12 Command change detection + --- 3.13 $(CC) support functions + --- 3.14 $(LD) support functions + --- 3.15 Script Invocation === 4 Host Program support --- 4.1 Simple Host Program @@ -321,7 +323,60 @@ more details, with real examples. names. This allows kbuild to totally skip the directory if the corresponding `CONFIG_` option is neither 'y' nor 'm'. -3.7 Compilation flags +3.7 Non-builtin vmlinux targets - extra-y +----------------------------------------- + + extra-y specifies targets which are needed for building vmlinux, + but not combined into built-in.a. + + Examples are: + + 1) head objects + + Some objects must be placed at the head of vmlinux. They are + directly linked to vmlinux without going through built-in.a + A typical use-case is an object that contains the entry point. + + arch/$(SRCARCH)/Makefile should specify such objects as head-y. + + Discussion: + Given that we can control the section order in our linker script, + why do we need head-y? + + 2) vmlinux linker script + + The linker script for vmlinux is located at + arch/$(SRCARCH)/kernel/vmlinux.lds + + Example:: + + # arch/x86/kernel/Makefile + extra-y := head_$(BITS).o + extra-y += head$(BITS).o + extra-y += ebda.o + extra-y += platform-quirks.o + extra-y += vmlinux.lds + + $(extra-y) should only contain targets needed for vmlinux. + + Kbuild skips extra-y when vmlinux is apparently not a final goal. + (e.g. 'make modules', or building external modules) + + If you intend to build targets unconditionally, always-y (explained + in the next section) is the correct syntax to use. + +3.8 Always built goals - always-y +--------------------------------- + + always-y specifies targets which are literally always built when + Kbuild visited the Makefile. + + Example:: + # ./Kbuild + offsets-file := include/generated/asm-offsets.h + always-y += $(offsets-file) + +3.9 Compilation flags --------------------- ccflags-y, asflags-y and ldflags-y @@ -410,8 +465,8 @@ more details, with real examples. AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt -3.8 Dependency tracking ------------------------ +3.10 Dependency tracking +------------------------ Kbuild tracks dependencies on the following: @@ -422,8 +477,8 @@ more details, with real examples. Thus, if you change an option to $(CC) all affected files will be re-compiled. -3.9 Custom Rules ----------------- +3.11 Custom Rules +----------------- Custom rules are used when the kbuild infrastructure does not provide the required support. A typical example is @@ -499,7 +554,7 @@ more details, with real examples. will be displayed with "make KBUILD_VERBOSE=0". -3.10 Command change detection +3.12 Command change detection ----------------------------- When the rule is evaluated, timestamps are compared between the target @@ -545,7 +600,7 @@ more details, with real examples. unwanted results when the target is up to date and only the tests on changed commands trigger execution of commands. -3.11 $(CC) support functions +3.13 $(CC) support functions ---------------------------- The kernel may be built with several different versions of @@ -660,7 +715,7 @@ more details, with real examples. endif endif -3.12 $(LD) support functions +3.14 $(LD) support functions ---------------------------- ld-option @@ -674,7 +729,7 @@ more details, with real examples. #Makefile LDFLAGS_vmlinux += $(call ld-option, -X) -3.13 Script invocation +3.15 Script invocation ---------------------- Make rules may invoke scripts to build the kernel. The rules shall @@ -1304,29 +1359,6 @@ When kbuild executes, the following steps are followed (roughly): When "make" is executed without arguments, bzImage will be built. -7.6 Building non-kbuild targets -------------------------------- - - extra-y - extra-y specifies additional targets created in the current - directory, in addition to any targets specified by `obj-*`. - - Listing all targets in extra-y is required for two purposes: - - 1) Enable kbuild to check changes in command lines - - - When $(call if_changed,xxx) is used - - 2) kbuild knows what files to delete during "make clean" - - Example:: - - #arch/x86/kernel/Makefile - extra-y := head.o init_task.o - - In this example, extra-y is used to list object files that - shall be built, but shall not be linked as part of built-in.a. - 7.7 Commands useful for building a boot image ---------------------------------------------