ANDROID: ion: flush cache before exporting non-cached buffers

The memory allocated from the page pool might be in the CPU cache, as the
zeroing can have been done through cache. e.g GFP_ZERO, or if the buffer
was used as cached memory previously. To map this memory as non-cached, we
need to flush the CPU cache first, to avoid any memory corruption when the
corresponding dirty cache line gets evicted.

A proper fix would be to flush the cache using the Linux DMA API. The
problem is that the buffer can be used without a device attached, which is
not covered since the introduction of:

  commit 1dccb598df ("arm64: simplify dma_get_ops")

This patch is then a dirty fix, calling directly cache flush function, to
cover the export of non-cached to an ION client. It ensures that the cache
line is actually cleaned before the memory is used as non-cached.

Change-Id: I9179c2f644a29ae71820c4ae63c880195f55e45d
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>

drivers/staging/android/ion/heaps/ion_cma_heap.c

@@ -70,6 +70,9 @@ static int ion_cma_allocate(struct ion_heap *heap, struct ion_buffer *buffer,
 
 	buffer->priv_virt = pages;
 	buffer->sg_table = table;
+
+	ion_buffer_prep_noncached(buffer);
+
 	return 0;
 
 free_mem:

drivers/staging/android/ion/heaps/ion_system_contig_heap.c

@@ -50,6 +50,8 @@ static int ion_system_contig_heap_allocate(struct ion_heap *heap,
 
 	buffer->sg_table = table;
 
+	ion_buffer_prep_noncached(buffer);
+
 	return 0;
 
 free_table:

drivers/staging/android/ion/heaps/ion_system_heap.c

@@ -141,6 +141,9 @@ static int ion_system_heap_allocate(struct ion_heap *heap,
 	}
 
 	buffer->sg_table = table;
+
+	ion_buffer_prep_noncached(buffer);
+
 	return 0;
 
 free_table:

drivers/staging/android/ion/ion_buffer.c

@@ -9,6 +9,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
+#include <linux/dma-noncoherent.h>
 
 #include "ion_private.h"
 
@@ -196,6 +197,24 @@ int ion_buffer_zero(struct ion_buffer *buffer)
 }
 EXPORT_SYMBOL_GPL(ion_buffer_zero);
 
+void ion_buffer_prep_noncached(struct ion_buffer *buffer)
+{
+	struct scatterlist *sg;
+	struct sg_table *table;
+	int i;
+
+	if (WARN_ONCE(!buffer || !buffer->sg_table,
+		      "%s needs a buffer and a sg_table", __func__) ||
+	    buffer->flags & ION_FLAG_CACHED)
+		return;
+
+	table = buffer->sg_table;
+
+	for_each_sg(table->sgl, sg, table->orig_nents, i)
+		arch_dma_prep_coherent(sg_page(sg), sg->length);
+}
+EXPORT_SYMBOL_GPL(ion_buffer_prep_noncached);
+
 void ion_buffer_release(struct ion_buffer *buffer)
 {
 	if (buffer->kmap_cnt > 0) {

include/linux/ion.h

@@ -279,6 +279,19 @@ int ion_heap_map_user(struct ion_heap *heap, struct ion_buffer *buffer,
  */
 int ion_buffer_zero(struct ion_buffer *buffer);
 
+/**
+ * ion_buffer_prep_noncached - flush cache before non-cached mapping
+ *
+ * @buffer:		ion_buffer to flush
+ *
+ * The memory allocated by the heap could be in the CPU cache. To map
+ * this memory as non-cached, we need to flush the associated cache
+ * first. Without the flush, it is possible for stale dirty cache lines
+ * to be evicted after the ION client started writing into this buffer,
+ * leading to data corruption.
+ */
+void ion_buffer_prep_noncached(struct ion_buffer *buffer);
+
 /**
  * ion_alloc - Allocates an ion buffer of given size from given heap
  *
@@ -358,6 +371,8 @@ static inline int ion_buffer_zero(struct ion_buffer *buffer)
 	return -EINVAL;
 }
 
+static inline void ion_buffer_prep_noncached(struct ion_buffer *buffer) {}
+
 static inline struct dma_buf *ion_alloc(size_t len, unsigned int heap_id_mask,
 					unsigned int flags)
 {