asm-generic: ioremap_uc should behave the same with and without MMU

Whatever reason there is for the existence of ioremap_uc, and the fact
that it returns NULL by default on architectures with an MMU applies
equally to nommu architectures, so don't provide different defaults.

In practice the difference is meaningless as the only portable driver
that uses ioremap_uc is atyfb which probably doesn't show up on nommu
devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Palmer Dabbelt <palmer@dabbelt.com>

include/asm-generic/io.h

@@ -935,18 +935,7 @@ static inline void *phys_to_virt(unsigned long address)
  * defined your own ioremap_*() variant you must then declare your own
  * ioremap_*() variant as defined to itself to avoid the default NULL return.
  */
-
-#ifdef CONFIG_MMU
-
-#ifndef ioremap_uc
-#define ioremap_uc ioremap_uc
-static inline void __iomem *ioremap_uc(phys_addr_t offset, size_t size)
-{
-	return NULL;
-}
-#endif
-
-#else /* !CONFIG_MMU */
+#ifndef CONFIG_MMU
 
 /*
  * Change "struct page" to physical address.
@@ -980,14 +969,6 @@ static inline void __iomem *ioremap_nocache(phys_addr_t offset, size_t size)
 }
 #endif
 
-#ifndef ioremap_uc
-#define ioremap_uc ioremap_uc
-static inline void __iomem *ioremap_uc(phys_addr_t offset, size_t size)
-{
-	return ioremap_nocache(offset, size);
-}
-#endif
-
 #ifndef ioremap_wc
 #define ioremap_wc ioremap_wc
 static inline void __iomem *ioremap_wc(phys_addr_t offset, size_t size)
@@ -1004,6 +985,21 @@ static inline void __iomem *ioremap_wt(phys_addr_t offset, size_t size)
 }
 #endif
 
+/*
+ * ioremap_uc is special in that we do require an explicit architecture
+ * implementation.  In general you do not want to use this function in a
+ * driver and use plain ioremap, which is uncached by default.  Similarly
+ * architectures should not implement it unless they have a very good
+ * reason.
+ */
+#ifndef ioremap_uc
+#define ioremap_uc ioremap_uc
+static inline void __iomem *ioremap_uc(phys_addr_t offset, size_t size)
+{
+	return NULL;
+}
+#endif
+
 #ifdef CONFIG_HAS_IOPORT_MAP
 #ifndef CONFIG_GENERIC_IOMAP
 #ifndef ioport_map