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Date:   Sun, 20 Sep 2020 12:42:51 +0900
From:   Masahiro Yamada <>
To:     Andrew Lunn <>
Cc:     netdev <>,
        David Miller <>,
        Jakub Kicinski <>,
        Michal Marek <>,
        Linux Kbuild mailing list <>,
        clang-built-linux <>
Subject: Re: [PATCH RFC/RFT 0/2] W=1 by default for Ethernet PHY subsystem

On Sun, Sep 20, 2020 at 4:03 AM Andrew Lunn <> wrote:
> There is a movement to make the code base compile clean with W=1. Some
> subsystems are already clean. In order to keep them clean, we need
> developers to build new code with W=1 by default in these subsystems.
> This patchset refactors the core Makefile warning code to allow the
> additional warnings W=1 adds available to any Makefile. The Ethernet
> PHY subsystem Makefiles then make use of this to make W=1 the default
> for this subsystem.
> RFT since i've only tested with x86 and arm with a modern gcc. Is the
> code really clean for older compilers? For clang?

I appreciate your efforts for keeping your subsystems
clean for W=1 builds, and I hope this work will be
extended towards upper directory level,
drivers/net/phy -> drivers/net -> drivers/.

However, when we talk about W=1, we consider not only the current
option set in W=1, but also options that might be added
by future compilers because every GCC/Clang
release adds new warning options.

Let's say, the future release, GCC 14 would
add a new option -Wfoo-bar, which is
reasonable enough to be enabled by default,
but doing so would emit a lot of warnings
in the current kernel tree.

We cannot add -Wfoo-bar to W=0 right away,
because our general consensus is that
the normal build should be warning-free.

In the current routine, we add -Wfoo-bar to W=1
with hope we can gradually fix the code and
eventually migrate it to W=0.
It is not always easy to move W=1 options to W=0
when we have lots of code fixed.
At least, 0-day bot iterates compile tests with W=1,
so new code violating -Wfoo-bar would be blocked.

With this patch series applied, where should we
add -Wfoo-bar? Adding it to W=1 would emit warnings
under drivers/net/ since W=1 is now the default
for the net subsystem.

Do we require to fix the code under driver/net/ first?
Or, should we add it to W=2 temporarily, then move it to W=1
once we fix drivers/net/?

So, another idea might be hard-coding extra warnings
like drivers/gpu/drm/i915/Makefile.

For example, your subsystem already achieved
-Wmissing-declarations free.

You can add

   subdir-ccflags-y += -Wmissing-declarations

to drivers/net/phy/Makefile.

Once you fix all net drivers, you can move it to
the parent, drivers/net/Makefile.

Then, drivers/Makefile next, and if it reaches
the top directory level, we can move it to W=0.

Some W=1 options stay there just because we cannot
fix lots of code.
So, our code should be improved with regard to W=1
warnings, but we might need some clarification
about how to do it gradually.

Comments are appreciated.

> Andrew Lunn (2):
>   scripts: Makefile.extrawarn: Add W=1 warnings to a symbol
>   net: phylib: Enable W=1 by default
>  drivers/net/mdio/Makefile  |  3 +++
>  drivers/net/pcs/Makefile   |  3 +++
>  drivers/net/phy/Makefile   |  3 +++
>  scripts/Makefile.extrawarn | 33 ++++++++++++++++++---------------
>  4 files changed, 27 insertions(+), 15 deletions(-)
> --
> 2.28.0

Best Regards
Masahiro Yamada

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