drm/i915: enable W=1 warnings by default

We enable a bunch more compiler warnings than the kernel
defaults. However, they've drifted to become a unique set of warnings,
and have increasingly fallen behind from the W=1 set.

Align with the W=1 warnings from scripts/Makefile.extrawarn for clarity,
by copy-pasting them with s/KBUILD_CFLAGS/subdir-ccflags-y/ to make it
easier to compare in the future.

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.

v2: Add back some -Wextra warning disables (Nathan)

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
[Final s/KBUILD_CFLAGS/subdir-ccflags-y/ fix while applying]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/69a812273091b6535ddc7f9346289d71bb30f43d.1697009258.git.jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile

@@ -3,22 +3,34 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
-# Add a set of useful warning flags and enable -Werror for CI to prevent
-# trivial mistakes from creeping in. We have to do this piecemeal as we reject
-# any patch that isn't warning clean, so turning on -Wextra (or W=1) we
-# need to filter out dubious warnings.  Still it is our interest
-# to keep running locally with W=1 C=1 until we are completely clean.
-#
-# Note the danger in using -Wextra is that when CI updates gcc we
-# will most likely get a sudden build breakage... Hopefully we will fix
-# new warnings before CI updates!
-subdir-ccflags-y := -Wextra
-subdir-ccflags-y += -Wno-unused-parameter
-subdir-ccflags-y += -Wno-type-limits
+# 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)
+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)
+subdir-ccflags-y += $(call cc-option, -Wformat-truncation)
+subdir-ccflags-y += $(call cc-option, -Wstringop-overflow)
+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-sign-compare
+subdir-ccflags-y += -Wno-type-limits
 subdir-ccflags-y += -Wno-shift-negative-value
-subdir-ccflags-y += $(call cc-option, -Wunused-but-set-variable)
+endif
+ifeq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
+subdir-ccflags-y += -Wno-sign-compare
+endif
+# --- end copy-paste
+
+# Enable -Werror in CI and development
 subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
 
 # Fine grained warnings disable