Re: [PATCH] kbuild: drop FORCE from PHONY targets

[Masahiro Yamada]

Hi Andy,


2016-03-14 13:44 GMT+09:00 Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
> On Sun, Mar 13, 2016 at 9:36 PM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>> 2016-03-14 13:28 GMT+09:00 Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
>>> On Sun, Mar 13, 2016 at 9:08 PM, Masahiro Yamada
>>> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>>>>
>>>> Hi Andy
>>>>
>>>> 2016-03-14 9:39 GMT+09:00 Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
>>>> > On Mar 12, 2016 4:14 PM, "Masahiro Yamada"
>>>> > <yamada.masahiro@xxxxxxxxxxxxx> 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@beagle:~/workspace/linux$ git describe
v4.5



Example 1)


Add the following code to the top Makefile.

Please note "FORCE" is used here.


yamada@beagle:~/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@beagle:~/workspace/linux$ git clean -x -f
yamada@beagle:~/workspace/linux$ make -s defconfig
yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo
scripts/kconfig/conf  --silentoldconfig Kconfig
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo
make: `foo' is up to date.
yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo
make: `foo' is up to date.
yamada@beagle:~/workspace/linux$ make GREETING=GoodEvening foo
  BAR     bar
  FOO     foo
yamada@beagle:~/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@beagle:~/workspace/linux$ git clean -x -f
yamada@beagle:~/workspace/linux$ make -s defconfig
yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo
scripts/kconfig/conf  --silentoldconfig Kconfig
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo
  BAR     bar
  FOO     foo
yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo
  BAR     bar
  FOO     foo
yamada@beagle:~/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