powerpc/32: Don't use a struct based type for pte_t

Long time ago we had a config item called STRICT_MM_TYPECHECKS
to build the kernel with pte_t defined as a structure in order
to perform additional build checks or build it with pte_t
defined as a simple type in order to get simpler generated code.

Commit 670eea92 ("powerpc/mm: Always use STRICT_MM_TYPECHECKS")
made the struct based definition the only one, considering that the
generated code was similar in both cases.

That's right on ppc64 because the ABI is such that the content of a
struct having a single simple type element is passed as register,
but on ppc32 such a structure is passed via the stack like any
structure.

Simple test function:

	pte_t test(pte_t pte)
	{
		return pte;
	}

Before this patch we get

	c00108ec <test>:
	c00108ec:	81 24 00 00 	lwz     r9,0(r4)
	c00108f0:	91 23 00 00 	stw     r9,0(r3)
	c00108f4:	4e 80 00 20 	blr

So, for PPC32, restore the simple type behaviour we got before
commit 670eea92, but instead of adding a config option to
activate type check, do it when __CHECKER__ is set so that type
checking is performed by 'sparse' and provides feedback like:

	arch/powerpc/mm/pgtable.c:466:16: warning: incorrect type in return expression (different base types)
	arch/powerpc/mm/pgtable.c:466:16:    expected unsigned long
	arch/powerpc/mm/pgtable.c:466:16:    got struct pte_t [usertype] x

With this patch we now get

	c0010890 <test>:
	c0010890:	4e 80 00 20 	blr
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Define STRICT_MM_TYPECHECKS rather than repeating the condition]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c904599f33aaf6bb7ee2836a9ff8368509e0d78d.1631887042.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/32/pgtable.h

@@ -245,7 +245,7 @@ static int number_of_cells_per_pte(pmd_t *pmd, pte_basic_t val, int huge)
 static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, pte_t *p,
 				     unsigned long clr, unsigned long set, int huge)
 {
-	pte_basic_t *entry = &p->pte;
+	pte_basic_t *entry = (pte_basic_t *)p;
 	pte_basic_t old = pte_val(*p);
 	pte_basic_t new = (old & ~(pte_basic_t)clr) | set;
 	int num, i;

arch/powerpc/include/asm/pgtable-types.h

@@ -2,17 +2,33 @@
 #ifndef _ASM_POWERPC_PGTABLE_TYPES_H
 #define _ASM_POWERPC_PGTABLE_TYPES_H
 
+#if defined(__CHECKER__) || !defined(CONFIG_PPC32)
+#define STRICT_MM_TYPECHECKS
+#endif
+
 /* PTE level */
 #if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
 typedef struct { pte_basic_t pte, pte1, pte2, pte3; } pte_t;
-#else
+#elif defined(STRICT_MM_TYPECHECKS)
 typedef struct { pte_basic_t pte; } pte_t;
+#else
+typedef pte_basic_t pte_t;
 #endif
+
+#if defined(STRICT_MM_TYPECHECKS) || \
+    (defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES))
 #define __pte(x)	((pte_t) { (x) })
 static inline pte_basic_t pte_val(pte_t x)
 {
 	return x.pte;
 }
+#else
+#define __pte(x)	((pte_t)(x))
+static inline pte_basic_t pte_val(pte_t x)
+{
+	return x;
+}
+#endif
 
 /* PMD level */
 #ifdef CONFIG_PPC64

arch/powerpc/mm/pgtable.c

@@ -271,7 +271,7 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep, pte_
 {
 	pmd_t *pmd = pmd_off(mm, addr);
 	pte_basic_t val;
-	pte_basic_t *entry = &ptep->pte;
+	pte_basic_t *entry = (pte_basic_t *)ptep;
 	int num, i;
 
 	/*