[PATCH 2/7] backports: Add fallthrough attribute

[Hauke Mehrtens <hauke@hauke-m.de>]

This adds the fallthrough attribute from Linux commit 294f69e662d1
("compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use")
This also adds the __has_attribute() define for older compilers like
GCC < 5.

linux/compiler_attributes.h needs to be included, this file was only
added in more recent kernel versions.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
---
 backport/backport-include/linux/compiler.h    |  2 ++
 .../linux/compiler_attributes.h               | 34 +++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 backport/backport-include/linux/compiler_attributes.h

diff --git a/backport/backport-include/linux/compiler.h b/backport/backport-include/linux/compiler.h
index 53c069df..51a6ec2c 100644
--- a/backport/backport-include/linux/compiler.h
+++ b/backport/backport-include/linux/compiler.h
@@ -2,6 +2,8 @@
 #define __BACKPORT_LINUX_COMPILER_H
 #include_next <linux/compiler.h>
 
+#include <linux/compiler_attributes.h>
+
 #ifndef __rcu
 #define __rcu
 #endif
diff --git a/backport/backport-include/linux/compiler_attributes.h b/backport/backport-include/linux/compiler_attributes.h
new file mode 100644
index 00000000..31ddc163
--- /dev/null
+++ b/backport/backport-include/linux/compiler_attributes.h
@@ -0,0 +1,34 @@
+#ifndef _BACKPORTS_LINUX_COMPILER_ATTRIBUTES_H
+#define _BACKPORTS_LINUX_COMPILER_ATTRIBUTES_H 1
+
+#if LINUX_VERSION_IS_GEQ(4,20,0)
+#include_next <linux/compiler_attributes.h>
+#endif
+
+#ifndef __has_attribute
+# define __has_attribute(x) __GCC4_has_attribute_##x
+#endif
+
+#ifndef __GCC4_has_attribute___fallthrough__
+# define __GCC4_has_attribute___fallthrough__         0
+#endif /* __GCC4_has_attribute___fallthrough__ */
+
+#ifndef fallthrough
+/*
+ * Add the pseudo keyword 'fallthrough' so case statement blocks
+ * must end with any of these keywords:
+ *   break;
+ *   fallthrough;
+ *   goto <label>;
+ *   return [expression];
+ *
+ *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
+ */
+#if __has_attribute(__fallthrough__)
+# define fallthrough                    __attribute__((__fallthrough__))
+#else
+# define fallthrough                    do {} while (0)  /* fallthrough */
+#endif
+#endif /* fallthrough */
+
+#endif /* _BACKPORTS_LINUX_COMPILER_ATTRIBUTES_H */
-- 
2.20.1

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