MIPS: DMA: For BMIPS5000 cores flush region just like non-coherent R10000

The BMIPS5000 (Zephyr) processor utilizes instruction speculation. A
stale misprediction address in either the JTB or the CRS may trigger
a prefetch inside a region that is currently being used by a DMA engine,
which is not IO-coherent.  This prefetch will fetch a line into the
scache, and that line will soon become stale (ie wrong) during/after the
DMA.  Mayhem ensues.

In dma-default.c, the r10000 is handled as a special case in the same way
that we want to handle Zephyr.  So we generalize the exception cases into
a function, and include Zephyr as one of the processors that needs this
special care.

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/5776/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/dma-default.c

@@ -50,16 +50,20 @@ static inline struct page *dma_addr_to_page(struct device *dev,
 }
 
 /*
+ * The affected CPUs below in 'cpu_needs_post_dma_flush()' can
+ * speculatively fill random cachelines with stale data at any time,
+ * requiring an extra flush post-DMA.
+ *
  * Warning on the terminology - Linux calls an uncached area coherent;
  * MIPS terminology calls memory areas with hardware maintained coherency
  * coherent.
  */
-
-static inline int cpu_is_noncoherent_r10000(struct device *dev)
+static inline int cpu_needs_post_dma_flush(struct device *dev)
 {
 	return !plat_device_is_coherent(dev) &&
 	       (current_cpu_type() == CPU_R10000 ||
-	       current_cpu_type() == CPU_R12000);
+		current_cpu_type() == CPU_R12000 ||
+		current_cpu_type() == CPU_BMIPS5000);
 }
 
 static gfp_t massage_gfp_flags(const struct device *dev, gfp_t gfp)
@@ -230,7 +234,7 @@ static inline void __dma_sync(struct page *page,
 static void mips_dma_unmap_page(struct device *dev, dma_addr_t dma_addr,
 	size_t size, enum dma_data_direction direction, struct dma_attrs *attrs)
 {
-	if (cpu_is_noncoherent_r10000(dev))
+	if (cpu_needs_post_dma_flush(dev))
 		__dma_sync(dma_addr_to_page(dev, dma_addr),
 			   dma_addr & ~PAGE_MASK, size, direction);
 
@@ -284,7 +288,7 @@ static void mips_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
 static void mips_dma_sync_single_for_cpu(struct device *dev,
 	dma_addr_t dma_handle, size_t size, enum dma_data_direction direction)
 {
-	if (cpu_is_noncoherent_r10000(dev))
+	if (cpu_needs_post_dma_flush(dev))
 		__dma_sync(dma_addr_to_page(dev, dma_handle),
 			   dma_handle & ~PAGE_MASK, size, direction);
 }
@@ -305,7 +309,7 @@ static void mips_dma_sync_sg_for_cpu(struct device *dev,
 
 	/* Make sure that gcc doesn't leave the empty loop body.  */
 	for (i = 0; i < nelems; i++, sg++) {
-		if (cpu_is_noncoherent_r10000(dev))
+		if (cpu_needs_post_dma_flush(dev))
 			__dma_sync(sg_page(sg), sg->offset, sg->length,
 				   direction);
 	}