ARC: dma [non IOC]: fix arc_dma_sync_single_for_(device|cpu)

ARC backend for dma_sync_single_for_(device|cpu) was broken as it was
not honoring the @dir argument and simply forcing it based on the call:
 - arc_dma_sync_single_for_device(dir) assumed DMA_TO_DEVICE (cache wback)
 - arc_dma_sync_single_for_cpu(dir) assumed DMA_FROM_DEVICE (cache inv)

This is not true given the DMA API programming model and has been
discussed here [1] in some detail.

Interestingly while the deficiency has been there forever, it only started
showing up after 4.17 dma common ops rework, commit a8eb92d0
("arc: fix arc_dma_{map,unmap}_page") which wired up these calls under the
more commonly used dma_map_page API triggering the issue.

[1]: https://lkml.org/lkml/2018/5/18/979
Fixes: commit a8eb92d0 ("arc: fix arc_dma_{map,unmap}_page")
Cc: stable@kernel.org # v4.17+
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
[vgupta: reworked changelog]

arch/arc/mm/dma.c

@@ -129,14 +129,59 @@ int arch_dma_mmap(struct device *dev, struct vm_area_struct *vma,
 	return ret;
 }
 
+/*
+ * Cache operations depending on function and direction argument, inspired by
+ * https://lkml.org/lkml/2018/5/18/979
+ * "dma_sync_*_for_cpu and direction=TO_DEVICE (was Re: [PATCH 02/20]
+ * dma-mapping: provide a generic dma-noncoherent implementation)"
+ *
+ *          |   map          ==  for_device     |   unmap     ==  for_cpu
+ *          |----------------------------------------------------------------
+ * TO_DEV   |   writeback        writeback      |   none          none
+ * FROM_DEV |   invalidate       invalidate     |   invalidate*   invalidate*
+ * BIDIR    |   writeback+inv    writeback+inv  |   invalidate    invalidate
+ *
+ *     [*] needed for CPU speculative prefetches
+ *
+ * NOTE: we don't check the validity of direction argument as it is done in
+ * upper layer functions (in include/linux/dma-mapping.h)
+ */
+
 void arch_sync_dma_for_device(struct device *dev, phys_addr_t paddr,
 		size_t size, enum dma_data_direction dir)
 {
+	switch (dir) {
+	case DMA_TO_DEVICE:
 		dma_cache_wback(paddr, size);
+		break;
+
+	case DMA_FROM_DEVICE:
+		dma_cache_inv(paddr, size);
+		break;
+
+	case DMA_BIDIRECTIONAL:
+		dma_cache_wback_inv(paddr, size);
+		break;
+
+	default:
+		break;
+	}
 }
 
 void arch_sync_dma_for_cpu(struct device *dev, phys_addr_t paddr,
 		size_t size, enum dma_data_direction dir)
 {
+	switch (dir) {
+	case DMA_TO_DEVICE:
+		break;
+
+	/* FROM_DEVICE invalidate needed if speculative CPU prefetch only */
+	case DMA_FROM_DEVICE:
+	case DMA_BIDIRECTIONAL:
 		dma_cache_inv(paddr, size);
+		break;
+
+	default:
+		break;
+	}
 }