[PATCH -fixes] dma-mapping: add default implementation to arch_dma_{set|clear}_uncached

[Yangyu Chen <cyy@cyyself.name>]

Currently, we have some code in kernel/dma/direct.c which references
arch_dma_set_uncached and arch_dma_clear_uncached. However, many
architectures do not provide these symbols, and the code currently
relies on compiler optimization to cut the unnecessary code. When the
compiler fails to optimize it, the code will reference the symbol and
cause a link error. I found this bug when developing some new extensions
for RISC-V on LLVM. The error message is shown below:

```
  LD      .tmp_vmlinux.kallsyms1
ld.lld: error: undefined symbol: arch_dma_set_uncached
>>> referenced by direct.c
>>>               kernel/dma/direct.o:(dma_direct_alloc) in archive
vmlinux.a
make[2]: *** [scripts/Makefile.vmlinux:34: vmlinux] Error 1
```

This patch adds default implementations for arch_dma_set_uncached and
arch_dma_clear_uncached in include/linux/dma-map-ops.h. So that the
code in kernel/dma/direct.c can always reference these symbols to
avoid link errors.

Signed-off-by: Yangyu Chen <cyy@cyyself.name>
---
 include/linux/dma-map-ops.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
index 02a1c825896b..92a713557859 100644
--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h
@@ -420,8 +420,22 @@ static inline void arch_dma_mark_clean(phys_addr_t paddr, size_t size)
 }
 #endif /* ARCH_HAS_DMA_MARK_CLEAN */
 
+#ifdef CONFIG_ARCH_HAS_DMA_SET_UNCACHED
 void *arch_dma_set_uncached(void *addr, size_t size);
+#else
+static inline void *arch_dma_set_uncached(void *addr, size_t size)
+{
+	return ERR_PTR(-EINVAL);
+}
+#endif /* CONFIG_ARCH_HAS_DMA_SET_UNCACHED */
+
+#ifdef CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED
 void arch_dma_clear_uncached(void *addr, size_t size);
+#else
+static inline void arch_dma_clear_uncached(void *addr, size_t size)
+{
+}
+#endif /* CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED */
 
 #ifdef CONFIG_ARCH_HAS_DMA_MAP_DIRECT
 bool arch_dma_map_page_direct(struct device *dev, phys_addr_t addr);
-- 
2.45.2