doc: use KCFLAGS instead of EXTRA_CFLAGS to pass flags from command line

You should use KCFLAGS to pass additional compiler flags from the
command line. Using EXTRA_CFLAGS is wrong.

EXTRA_CFLAGS is supposed to specify flags applied only to the current
Makefile (and now deprecated in favor of ccflags-y).

It is still used in arch/mips/kvm/Makefile (and possibly in external
modules too). Passing EXTRA_CFLAGS from the command line overwrites
it and breaks the build.

I also fixed drivers/gpu/drm/tilcdc/Makefile because commit 816175dd1fd7
("drivers/gpu/drm/tilcdc: Makefile, only -Werror when no -W* in
EXTRA_CFLAGS") was based on the same misunderstanding.

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Alex Shi <[e-mail]>
Acked-by: Federico Vaga <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/process/4.Coding.rst

···
 its cause.
 
 Note that not all compiler warnings are enabled by default.  Build the
-kernel with "make EXTRA_CFLAGS=-W" to get the full set.
+kernel with "make KCFLAGS=-W" to get the full set.
 
 The kernel provides several configuration options which turn on debugging
 features; most of these are found in the "kernel hacking" submenu.  Several

Documentation/process/submit-checklist.rst

···
     injection might be appropriate.
 
 20) Newly-added code has been compiled with ``gcc -W`` (use
-    ``make EXTRA_CFLAGS=-W``).  This will generate lots of noise, but is good
+    ``make KCFLAGS=-W``).  This will generate lots of noise, but is good
     for finding bugs like "warning: comparison between signed and unsigned".
 
 21) Tested after it has been merged into the -mm patchset to make sure

Documentation/translations/it_IT/process/4.Coding.rst

···
 però averne trovato la causa.
 
 Tenete a mente che non tutti gli avvertimenti sono disabilitati di default.
-Costruite il kernel con "make EXTRA_CFLAGS=-W" per ottenerli tutti.
+Costruite il kernel con "make KCFLAGS=-W" per ottenerli tutti.
 
 Il kernel fornisce differenti opzioni che abilitano funzionalità di debugging;
 molti di queste sono trovano all'interno del sotto menu "kernel hacking".

Documentation/translations/it_IT/process/submit-checklist.rst

···
     l'iniezione di fallimenti specifici per il sottosistema.
 
 22) Il nuovo codice è stato compilato con ``gcc -W`` (usate
-    ``make EXTRA_CFLAGS=-W``).  Questo genererà molti avvisi, ma è ottimo
+    ``make KCFLAGS=-W``).  Questo genererà molti avvisi, ma è ottimo
     per scovare bachi come  "warning: comparison between signed and unsigned".
 
 23) La patch è stata verificata dopo essere stata inclusa nella serie di patch

Documentation/translations/zh_CN/process/4.Coding.rst

···
 通常，这些警告都指向真正的问题。提交以供审阅的代码通常不会产生任何编译器警告。
 在消除警告时，注意了解真正的原因，并尽量避免“修复”，使警告消失而不解决其原因。
 
-请注意，并非所有编译器警告都默认启用。使用“make EXTRA_CFLAGS=-W”构建内核以
+请注意，并非所有编译器警告都默认启用。使用“make KCFLAGS=-W”构建内核以
 获得完整集合。
 
 内核提供了几个配置选项，可以打开调试功能；大多数配置选项位于“kernel hacking”

drivers/gpu/drm/tilcdc/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
-ifeq (, $(findstring -W,$(EXTRA_CFLAGS)))
+ifeq (, $(findstring -W,$(KCFLAGS)))
 	ccflags-y += -Werror
 endif