Re: [PATCH 5/6] drm: enable (most) W=1 warnings by default across the subsystem

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 10 Jan 2024, Hamza Mahfooz <hamza.mahfooz@amd.com> wrote:
> On 1/10/24 12:39, Jani Nikula wrote:
>> At least the i915 and amd drivers enable a bunch more compiler warnings
>> than the kernel defaults.
>> 
>> Extend most of the W=1 warnings to the entire drm subsystem by
>> default. Use the copy-pasted warnings from scripts/Makefile.extrawarn
>> with s/KBUILD_CFLAGS/subdir-ccflags-y/ to make it easier to compare and
>> keep up with them in the future.
>> 
>> This is similar to the approach currently used in i915.
>> 
>> Some of the -Wextra warnings do need to be disabled, just like in
>> Makefile.extrawarn, but take care to not disable them for W=2 or W=3
>> builds, depending on the warning.
>> 
>> There are too many -Wformat-truncation warnings to cleanly fix up front;
>> leave that warning disabled for now.
>> 
>> v2:
>> - Drop -Wformat-truncation (too many warnings)
>> - Drop -Wstringop-overflow (enabled by default upstream)
>> 
>> Cc: David Airlie <airlied@gmail.com>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> Cc: Maxime Ripard <mripard@kernel.org>
>> Cc: Thomas Zimmermann <tzimmermann@suse.de>
>> Cc: Alex Deucher <alexander.deucher@amd.com>
>> Cc: Christian König <christian.koenig@amd.com>
>> Cc: Pan, Xinhui <Xinhui.Pan@amd.com>
>> Cc: Karol Herbst <kherbst@redhat.com>
>> Cc: Lyude Paul <lyude@redhat.com>
>> Cc: Danilo Krummrich <dakr@redhat.com>
>> Cc: Rob Clark <robdclark@gmail.com>
>> Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
>> Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
>> Cc: Sean Paul <sean@poorly.run>
>> Cc: Marijn Suijten <marijn.suijten@somainline.org>
>> Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
>> Acked-by: Javier Martinez Canillas <javierm@redhat.com>
>> Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>> ---
>>   drivers/gpu/drm/Makefile | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>> 
>> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
>> index 104b42df2e95..8b6be830f7c3 100644
>> --- a/drivers/gpu/drm/Makefile
>> +++ b/drivers/gpu/drm/Makefile
>> @@ -5,6 +5,33 @@
>>   
>>   CFLAGS-$(CONFIG_DRM_USE_DYNAMIC_DEBUG)	+= -DDYNAMIC_DEBUG_MODULE
>>   
>> +# Unconditionally enable W=1 warnings locally
>> +# --- begin copy-paste W=1 warnings from scripts/Makefile.extrawarn
>> +subdir-ccflags-y += -Wextra -Wunused -Wno-unused-parameter
>> +subdir-ccflags-y += -Wmissing-declarations
>> +subdir-ccflags-y += $(call cc-option, -Wrestrict)
>
> It would be safer to do something along the lines of:
>
> cond-flags := $(call cc-option, -Wrestrict) \
> 	$(call cc-option, -Wunused-but-set-variable) \
> 	$(call cc-option, -Wunused-const-variable) \
> 	$(call cc-option, -Wunused-const-variable) \
> 	$(call cc-option, -Wformat-overflow) \
> 	$(call cc-option, -Wstringop-truncation)
> subdir-ccflags-y += $(cond-flags)
>
> Otherwise, you will end up breaking `$ make M=drivers/gpu/drm`
> for a bunch of people.

I discussed this with Alex on IRC yesterday. The above seems obviously
correct in that it just changes the evaluation time of $(call cc-option,
...). Apparently not having it may lead to:

scripts/Makefile.lib:10: *** Recursive variable 'KBUILD_CFLAGS' references itself (eventually).  Stop.

Of course, I could just throw that in and be happy, but me being me, I'd
really like to know what is going on here first. :)

For one thing, I always thought M=dir was only for out-of-tree modules,
though the IRC discussion seems to indicate a lot of people also use it
for in-tree modules. But I can't even make it to work for a lot of cases
on top of current drm-tip, without the changes here.

M=drivers/gpu/drm/i915 fails immediately. So does
M=drivers/gpu/drm/amd. And M=drivers/gpu/drm/nouveau. And
M=drivers/gpu/drm/radeon.

M=drivers/gpu/drm fails with the same cases as above. I always use
KBUILD_OUTPUT=dir but I don't know if it makes a difference, I didn't
try.

However M=drivers/gpu/drm/amd/amdgpu works.

The only way I could reproduce the "recursive variable" issue in that
was using:

subdir-ccflags-y = -Wextra

instead of:

subdir-ccflags-y := -Wextra

or:

subdir-ccflags-y += -Wextra

in amdgpu/Makefile

Since I use the latter form in this pach, I think it should be fine for
M=dir users even if M=dir doesn't really seem to generally work for
in-tree modules (at least not for me).

Cc: Masahiro


BR,
Jani.


>
>> +subdir-ccflags-y += -Wmissing-format-attribute
>> +subdir-ccflags-y += -Wmissing-prototypes
>> +subdir-ccflags-y += -Wold-style-definition
>> +subdir-ccflags-y += -Wmissing-include-dirs
>> +subdir-ccflags-y += $(call cc-option, -Wunused-but-set-variable)
>> +subdir-ccflags-y += $(call cc-option, -Wunused-const-variable)
>> +subdir-ccflags-y += $(call cc-option, -Wpacked-not-aligned)
>> +subdir-ccflags-y += $(call cc-option, -Wformat-overflow)
>> +# FIXME: fix -Wformat-truncation warnings and uncomment
>> +#subdir-ccflags-y += $(call cc-option, -Wformat-truncation)
>> +subdir-ccflags-y += $(call cc-option, -Wstringop-truncation)
>> +# The following turn off the warnings enabled by -Wextra
>> +ifeq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
>> +subdir-ccflags-y += -Wno-missing-field-initializers
>> +subdir-ccflags-y += -Wno-type-limits
>> +subdir-ccflags-y += -Wno-shift-negative-value
>> +endif
>> +ifeq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
>> +subdir-ccflags-y += -Wno-sign-compare
>> +endif
>> +# --- end copy-paste
>> +
>>   drm-y := \
>>   	drm_aperture.o \
>>   	drm_atomic.o \

-- 
Jani Nikula, Intel