linux-kernel - [PATCH] kbuild: Disable -Wpointer-to-enum-cast

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Date:   Sun,  8 Mar 2020 00:34:00 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        stable@...r.kernel.org
Subject: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

Clang's -Wpointer-to-int-cast deviates from GCC in that it warns when
casting to enums. The kernel does this in certain places, such as device
tree matches to set the version of the device being used, which allows
the kernel to avoid using a gigantic union.

https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L428
https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L402
https://elixir.bootlin.com/linux/v5.5.8/source/include/linux/mod_devicetable.h#L264

To avoid a ton of false positive warnings, disable this particular part
of the warning, which has been split off into a separate diagnostic so
that the entire warning does not need to be turned off for clang.

Cc: stable@...r.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/887
Link: https://github.com/llvm/llvm-project/commit/2a41b31fcdfcb67ab7038fc2ffb606fd50b83a84
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index 86035d866f2c..90e56d5657c9 100644
--- a/Makefile
+++ b/Makefile
@@ -748,6 +748,10 @@ KBUILD_CFLAGS += -Wno-tautological-compare
 # source of a reference will be _MergedGlobals and not on of the whitelisted names.
 # See modpost pattern 2
 KBUILD_CFLAGS += -mno-global-merge
+# clang's -Wpointer-to-int-cast warns when casting to enums, which does not match GCC.
+# Disable that part of the warning because it is very noisy across the kernel and does
+# not point out any real bugs.
+KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
 else
 
 # These warnings generated too much noise in a regular build.
-- 
2.25.1