Patch "ARC: mm: fix build breakage with STRICT_MM_TYPECHECKS" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ARC: mm: fix build breakage with STRICT_MM_TYPECHECKS

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arc-mm-fix-build-breakage-with-strict_mm_typechecks.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vineet Gupta <[email protected]>
Date: Tue, 16 Aug 2016 18:27:07 -0700
Subject: ARC: mm: fix build breakage with STRICT_MM_TYPECHECKS
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Vineet Gupta <[email protected]>

commit 1c3c909303924d30145601f47b6c058fdd2cbc2e upstream.

|  CC      mm/memory.o
| In file included from ../mm/memory.c:53:0:
| ../include/linux/pfn_t.h: In function ‘pfn_t_pte’:
| ../include/linux/pfn_t.h:78:2: error: conversion to non-scalar type requested
|  return pfn_pte(pfn_t_to_pfn(pfn), pgprot);

With STRICT_MM_TYPECHECKS pte_t is a struct and the offending code forces a cast which ends up shifting a struct and hence the gcc warning.

Note that in recent past some of the arches (aarch64, s390) made STRICT_MM_TYPECHECKS default, but we don't for ARC as this leads to slightly worse generated code, given ARC ABI definition of returning structs (which pte_t would become)

Quoting from ARC ABI...

"Results of type struct are returned in a caller-supplied temporary variable whose address is passed in r0.
For such functions, the arguments are shifted so that they are passed in r1 and up."

So
- struct to be returned would be allocated on stack requiring extra code at call sites
- callee updates stack memory to facilitate the return (vs. simple MOV into return reg r0)

Hence STRICT_MM_TYPECHECKS is not enabled by default for ARC

---
 arch/arc/include/asm/pgtable.h |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

@@ -277,8 +277,7 @@ static inline void pmd_set(pmd_t *pmdp,
 
 #define mk_pte(page, prot)	pfn_pte(page_to_pfn(page), prot)
 #define pte_pfn(pte)		(pte_val(pte) >> PAGE_SHIFT)
-#define pfn_pte(pfn, prot)	(__pte(((pte_t)(pfn) << PAGE_SHIFT) | \
-				 pgprot_val(prot)))
+#define pfn_pte(pfn, prot)	(__pte(((pfn) << PAGE_SHIFT) | pgprot_val(prot)))
 #define __pte_index(addr)	(((addr) >> PAGE_SHIFT) & (PTRS_PER_PTE - 1))
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/arc-mm-fix-build-breakage-with-strict_mm_typechecks.patch