s390: Remove ioremap_wt() and pgprot_writethrough()

It turns out that while s390 architecture calls its memory-I/O mapping
variants write-through and write-back the implementation of ioremap_wt()
and pgprot_writethrough() does not match Linux notion of ioremap_wt().

In particular Linux expects ioremap_wt() to be weaker still than
ioremap_wc(), allowing not just gathering and re-ordering but also reads
to be served from cache. Instead s390's implementation is equivalent to
normal ioremap() while its ioremap_wc() allows re-ordering.

Note that there are no known users of ioremap_wt() on s390 and the
resulting behavior is in line with asm-generic defining ioremap_wt() as
ioremap(), if undefined, so no breakage is expected.

As s390 does not have a mapping type matching the Linux notion of
ioremap_wt() and pgprot_writethrough(), simply drop them and rely on the
asm-generic fallbacks instead.

Fixes: b02002c ("s390/pci: Implement ioremap_wc/prot() with MIO")
Fixes: b43b3ff ("s390: mm: convert to GENERIC_IOREMAP")
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: niklas88

arch/s390/include/asm/io.h

@@ -34,8 +34,6 @@ void unxlate_dev_mem_ptr(phys_addr_t phys, void *addr);
 
 #define ioremap_wc(addr, size)  \
 	ioremap_prot((addr), (size), pgprot_val(pgprot_writecombine(PAGE_KERNEL)))
-#define ioremap_wt(addr, size)  \
-	ioremap_prot((addr), (size), pgprot_val(pgprot_writethrough(PAGE_KERNEL)))
 
 static inline void __iomem *ioport_map(unsigned long port, unsigned int nr)
 {

arch/s390/include/asm/pgtable.h

@@ -1403,9 +1403,6 @@ void gmap_pmdp_idte_global(struct mm_struct *mm, unsigned long vmaddr);
 #define pgprot_writecombine	pgprot_writecombine
 pgprot_t pgprot_writecombine(pgprot_t prot);
 
-#define pgprot_writethrough	pgprot_writethrough
-pgprot_t pgprot_writethrough(pgprot_t prot);
-
 #define PFN_PTE_SHIFT		PAGE_SHIFT
 
 /*

arch/s390/mm/pgtable.c

@@ -36,16 +36,6 @@ pgprot_t pgprot_writecombine(pgprot_t prot)
 }
 EXPORT_SYMBOL_GPL(pgprot_writecombine);
 
-pgprot_t pgprot_writethrough(pgprot_t prot)
-{
-	/*
-	 * mio_wb_bit_mask may be set on a different CPU, but it is only set
-	 * once at init and only read afterwards.
-	 */
-	return __pgprot(pgprot_val(prot) & ~mio_wb_bit_mask);
-}
-EXPORT_SYMBOL_GPL(pgprot_writethrough);
-
 static inline void ptep_ipte_local(struct mm_struct *mm, unsigned long addr,
 				   pte_t *ptep, int nodat)
 {