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Message-ID: <CAK7LNATAeMmfwWiZha17mbyWhF6bR5RGjv+gbsQTvL4teoTiPQ@mail.gmail.com>
Date: Mon, 14 Mar 2016 15:26:14 +0900
From: Masahiro Yamada <yamada.masahiro@...ionext.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Fenghua Yu <fenghua.yu@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Russell King <linux@....linux.org.uk>,
Michal Marek <mmarek@...e.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
Nathan Lynch <nathan_lynch@...tor.com>,
Tony Luck <tony.luck@...el.com>,
"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH] kbuild: drop FORCE from PHONY targets
Hi Andy,
2016-03-14 13:44 GMT+09:00 Andy Lutomirski <luto@...capital.net>:
> On Sun, Mar 13, 2016 at 9:36 PM, Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
>> 2016-03-14 13:28 GMT+09:00 Andy Lutomirski <luto@...capital.net>:
>>> On Sun, Mar 13, 2016 at 9:08 PM, Masahiro Yamada
>>> <yamada.masahiro@...ionext.com> wrote:
>>>>
>>>> Hi Andy
>>>>
>>>> 2016-03-14 9:39 GMT+09:00 Andy Lutomirski <luto@...capital.net>:
>>>> > On Mar 12, 2016 4:14 PM, "Masahiro Yamada"
>>>> > <yamada.masahiro@...ionext.com> wrote:
>>>> >>
>>>> >> These targets are marked as PHONY. No need to add FORCE to their
>>>> >> dependency.
>>>> >
>>>> > If this is, in fact, correct, can you update
>>>> > Documentation/kbuild/makefiles.txt as well?
>>>>
>>>> Which line do you want me to update?
>>>>
>>>
>>> All the references to FORCE should probably mention .PHONY as an alternative.
>>
>> I do not get your point.
>>
>> All the examples in the makefile.txt correctly reference to FORCE.
>> They are not PHONY targets.
>> No need to update.
>
> But they could be. For example:
>
> $(obj)/image: vmlinux FORCE
> $(call if_changed,objcopy)
>
> could be:
>
> .PHONY: $(obj)/image
> $(obj)/image: vmlinux
> $(call if_changed,objcopy)
>
> I would at least change:
>
> Note: It is a typical mistake to forget the FORCE prerequisite.
>
> to:
>
> Note: if-changed is only useful if make executes it, which won't
> happen if it determines that the inputs have not changed since the
> output was built. This can be avoided by specifying FORCE as a
> prerequisite or by making declaring the output as .PHONY.
>
No. This is absolutely wrong.
PHONY and FORCE are not interchangeable.
They have different behavior.
I will show you how they behave differently.
BTW, Linux 4.5 is out now.
Let's try a simple experiment on it.
The following shows that my source tree is v4.5
yamada@...gle:~/workspace/linux$ git describe
v4.5
Example 1)
Add the following code to the top Makefile.
Please note "FORCE" is used here.
yamada@...gle:~/workspace/linux$ git diff
diff --git a/Makefile b/Makefile
index 7b3ecdc..89b7d0d 100644
--- a/Makefile
+++ b/Makefile
@@ -914,6 +914,22 @@ export KBUILD_ALLDIRS := $(sort $(filter-out
arch/%,$(vmlinux-
vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)
+quiet_cmd_gen_foo = FOO $@
+ cmd_gen_foo = (cat $<; echo hello) > $@
+
+foo: bar FORCE
+ $(call if_changed,gen_foo)
+
+quiet_cmd_gen_bar = BAR $@
+ cmd_gen_bar = (cat $<; echo $(GREETING)) > $@
+
+bar: baz FORCE
+ $(call if_changed,gen_bar)
+
+baz:
+ @touch $@
+
+
# Final link of vmlinux
cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux)
quiet_cmd_link-vmlinux = LINK $@
Try the following.
yamada@...gle:~/workspace/linux$ git clean -x -f
yamada@...gle:~/workspace/linux$ make -s defconfig
yamada@...gle:~/workspace/linux$ make GREETING=GoodMorning foo
scripts/kconfig/conf --silentoldconfig Kconfig
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodMorning foo
make: `foo' is up to date.
yamada@...gle:~/workspace/linux$ make GREETING=GoodAfternoon foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodAfternoon foo
make: `foo' is up to date.
yamada@...gle:~/workspace/linux$ make GREETING=GoodEvening foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodEvening foo
make: `foo' is up to date.
Please notice "foo" and "bar" were not rebuilt
when I gave the same command line as the previous run.
When I changed the command line, "bar" was update
and "foo" was also updated because "foo" depends on "bar".
It means $(call if_changed,...) is working as expected.
Example 2)
Add the following to the top Makefile.
Please notice that I just replaced "FORCE" with ".PHONY".
diff --git a/Makefile b/Makefile
index 7b3ecdc..a0899c3 100644
--- a/Makefile
+++ b/Makefile
@@ -914,6 +914,24 @@ export KBUILD_ALLDIRS := $(sort $(filter-out
arch/%,$(vmlinux-
vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)
+quiet_cmd_gen_foo = FOO $@
+ cmd_gen_foo = (cat $<; echo hello) > $@
+
+.PHONY: foo
+foo: bar
+ $(call if_changed,gen_foo)
+
+quiet_cmd_gen_bar = BAR $@
+ cmd_gen_bar = (cat $<; echo $(GREETING)) > $@
+
+.PHONY: bar
+bar: baz
+ $(call if_changed,gen_bar)
+
+baz:
+ @touch $@
+
+
# Final link of vmlinux
cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux)
quiet_cmd_link-vmlinux = LINK $@
Let's try the same thing.
yamada@...gle:~/workspace/linux$ git clean -x -f
yamada@...gle:~/workspace/linux$ make -s defconfig
yamada@...gle:~/workspace/linux$ make GREETING=GoodMorning foo
scripts/kconfig/conf --silentoldconfig Kconfig
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodMorning foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodMorning foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodAfternoon foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodAfternoon foo
BAR bar
FOO foo
yamada@...gle:~/workspace/linux$ make GREETING=GoodAfternoon foo
BAR bar
FOO foo
Did you notice the difference?
The "foo" and "bar" are always rebuilt
regardless the command line is changed or not.
It means $(call if_changed,...) is not working.
--
Best Regards
Masahiro Yamada
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