mips: avoid explicit UB in assignment of mips_io_port_base

The code in question is modifying a variable declared const through
pointer manipulation.  Such code is explicitly undefined behavior, and
is the lone issue preventing malta_defconfig from booting when built
with Clang:

If an attempt is made to modify an object defined with a const-qualified
type through use of an lvalue with non-const-qualified type, the
behavior is undefined.

LLVM is removing such assignments. A simple fix is to not declare
variables const that you plan on modifying.  Limiting the scope would be
a better method of preventing unwanted writes to such a variable.

Further, the code in question mentions "compiler bugs" without any links
to bug reports, so it is difficult to know if the issue is resolved in
GCC. The patch was authored in 2006, which would have been GCC 4.0.3 or
4.1.1. The minimal supported version of GCC in the Linux kernel is
currently 4.6.

For what its worth, there was UB before the commit in question, it just
added a barrier and got lucky IRT codegen. I don't think there's any
actual compiler bugs related, just runtime bugs due to UB.

Link: https://github.com/ClangBuiltLinux/linux/issues/610
Fixes: 966f4406d903 ("[MIPS] Work around bad code generation for <asm/io.h>.")
Reported-by: Nathan Chancellor <[email protected]>
Debugged-by: Nathan Chancellor <[email protected]>
Suggested-by: Eli Friedman <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Paul Burton <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Maciej W. Rozycki <[email protected]>
Cc: Hassan Naveed <[email protected]>
Cc: Stephen Kitt <[email protected]>
Cc: Serge Semin <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]

0 files changed, 0 insertions, 0 deletions