[PATCH] Documentation: kbuild: Add note about using (subst m,y)

[PATCH] Documentation: kbuild: Add note about using (subst m,y)

[David Gow]

THe kbuild documentation already notes that, where there's an obj-y
target in a subdirectory, it will be orphaned if the subdirectory uses
obj-m. Suggest a way of forcing the directory to obj-y when it's based
on a config option which could be 'm'.

Linus made several suggestions as to how this is usually done here:
https://lore.kernel.org/linux-kselftest/CAHk-=wgK07PQ_DBBbAbSJ41t__6de1xZ6q7RRu-JS=2SU0wqLQ@mail.gmail.com/

This only documents the first one, which seems most common.

Signed-off-by: David Gow <davidgow@google.com>
---
 Documentation/kbuild/makefiles.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index e67eb261c9b0..d9eb24799f52 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -254,6 +254,17 @@ from that directory will be linked into vmlinux. If the Makefile in
 that directory specifies obj-y, those objects will be left orphan.
 It is very likely a bug of the Makefile or of dependencies in Kconfig.
 
+This means that, if a directory contains both obj-y and obj-m targets,
+it should be added with obj-y. If this directory nevertheless should
+only be built when a config option is enabled (typically to support
+a module which requires one or two files to nevertheless be built-in),
+'subst m,y' can be used to ensure obj-y is used.
+
+Example::
+
+  #drivers/Makefile
+  obj-$(subst m,y,$(CONFIG_HYPERV)) += hv/
+
 Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
 descending into subdirectories. It is a good fit when you know they
 do not contain kernel-space objects at all. A typical usage is to let
-- 
2.39.2.722.g9855ee24e9-goog

Re: [PATCH] Documentation: kbuild: Add note about using (subst m,y)

[Masahiro Yamada]

On Tue, Feb 28, 2023 at 12:13 PM David Gow <davidgow@google.com> wrote:
>
> THe kbuild documentation already notes that, where there's an obj-y
> target in a subdirectory, it will be orphaned if the subdirectory uses
> obj-m. Suggest a way of forcing the directory to obj-y when it's based
> on a config option which could be 'm'.
>
> Linus made several suggestions as to how this is usually done here:
> https://lore.kernel.org/linux-kselftest/CAHk-=wgK07PQ_DBBbAbSJ41t__6de1xZ6q7RRu-JS=2SU0wqLQ@mail.gmail.com/
>
> This only documents the first one, which seems most common.
>
> Signed-off-by: David Gow <davidgow@google.com>
> ---
>  Documentation/kbuild/makefiles.rst | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
> index e67eb261c9b0..d9eb24799f52 100644
> --- a/Documentation/kbuild/makefiles.rst
> +++ b/Documentation/kbuild/makefiles.rst
> @@ -254,6 +254,17 @@ from that directory will be linked into vmlinux. If the Makefile in
>  that directory specifies obj-y, those objects will be left orphan.
>  It is very likely a bug of the Makefile or of dependencies in Kconfig.
>
> +This means that, if a directory contains both obj-y and obj-m targets,
> +it should be added with obj-y. If this directory nevertheless should
> +only be built when a config option is enabled (typically to support
> +a module which requires one or two files to nevertheless be built-in),
> +'subst m,y' can be used to ensure obj-y is used.
> +
> +Example::
> +
> +  #drivers/Makefile
> +  obj-$(subst m,y,$(CONFIG_HYPERV)) += hv/
> +


I think many subsystems simply do

obj-y  += hv/


I do not think we need to advertise hyperv's way
since it does not look very pretty.







Mostly, it looks like this:


obj-y  += kunit/

and

obj-$(CONFIG_KUNIT_HOOK)   += hook.o







Bikeshed:

I think Linus' suggestion is OK, but
the BSD style seems less ugly,
of course, that is just a matter of style.


obj-$(CONFIG_HYPERV:m=y)   += kunit/




>  Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
>  descending into subdirectories. It is a good fit when you know they
>  do not contain kernel-space objects at all. A typical usage is to let
> --
> 2.39.2.722.g9855ee24e9-goog
>


--
Best Regards
Masahiro Yamada

Re: [PATCH] Documentation: kbuild: Add note about using (subst m,y)

[Alexander Lobakin]

From: Masahiro Yamada <masahiroy@kernel.org>
Date: Tue, 28 Feb 2023 17:30:09 +0900

> On Tue, Feb 28, 2023 at 12:13 PM David Gow <davidgow@google.com> wrote:

[...]

>> +Example::
>> +
>> +  #drivers/Makefile
>> +  obj-$(subst m,y,$(CONFIG_HYPERV)) += hv/
>> +
> 
> 
> I think many subsystems simply do
> 
> obj-y  += hv/

This creates a ton of empty built-in.a, each of them is listed in the
Kbuild output. Someone may think that if a directory contains
built-in.a, then something was built there. Sure it's their problems,
but I'd prefer to not pollute the log and built-in.a contents when
possible (empty files are still listed there IIRC).

> 
> 
> I do not think we need to advertise hyperv's way
> since it does not look very pretty.
> 
> 
> 
> 
> 
> 
> 
> Mostly, it looks like this:
> 
> 
> obj-y  += kunit/
> 
> and
> 
> obj-$(CONFIG_KUNIT_HOOK)   += hook.o
> 
> 
> 
> 
> 
> 
> 
> Bikeshed:
> 
> I think Linus' suggestion is OK, but
> the BSD style seems less ugly,
> of course, that is just a matter of style.
> 
> 
> obj-$(CONFIG_HYPERV:m=y)   += kunit/

I'd vote for this one, it's compact and readable.

> 
> 
> 
> 
>>  Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
>>  descending into subdirectories. It is a good fit when you know they
>>  do not contain kernel-space objects at all. A typical usage is to let
>> --
>> 2.39.2.722.g9855ee24e9-goog
>>
> 
> 
> --
> Best Regards
> Masahiro Yamada
> 

Thanks,
Olek

Re: [PATCH] Documentation: kbuild: Add note about using (subst m,y)

[Masahiro Yamada]

On Wed, Mar 1, 2023 at 12:16 AM Alexander Lobakin
<aleksander.lobakin@intel.com> wrote:
>
> From: Masahiro Yamada <masahiroy@kernel.org>
> Date: Tue, 28 Feb 2023 17:30:09 +0900
>
> > On Tue, Feb 28, 2023 at 12:13 PM David Gow <davidgow@google.com> wrote:
>
> [...]
>
> >> +Example::
> >> +
> >> +  #drivers/Makefile
> >> +  obj-$(subst m,y,$(CONFIG_HYPERV)) += hv/
> >> +
> >
> >
> > I think many subsystems simply do
> >
> > obj-y  += hv/
>
> This creates a ton of empty built-in.a, each of them is listed in the
> Kbuild output. Someone may think that if a directory contains
> built-in.a, then something was built there. Sure it's their problems,
> but I'd prefer to not pollute the log and built-in.a contents when
> possible (empty files are still listed there IIRC).


You can choose whichever you like.
Up to each subsystem maintainer.






-- 
Best Regards
Masahiro Yamada