Split out a bunch of a self-contained helpers to make the function easier
to follow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 kernel/dma/swiotlb.c | 179 +++++++++++++++++++++----------------------
 1 file changed, 89 insertions(+), 90 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index b38b1553c4663a..381c24ef1ac1d0 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c

@@ -468,134 +468,133 @@ static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,

 	}
 }
 

-phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t orig_addr,
-		size_t mapping_size, size_t alloc_size,
-		enum dma_data_direction dir, unsigned long attrs)
-{
-	dma_addr_t tbl_dma_addr = phys_to_dma_unencrypted(hwdev, io_tlb_start);
-	unsigned long flags;
-	phys_addr_t tlb_addr;
-	unsigned int nslots, stride, index, wrap;
-	int i;
-	unsigned long mask;
-	unsigned long offset_slots;
-	unsigned long max_slots;
-	unsigned long tmp_io_tlb_used;
-
-	if (no_iotlb_memory)
-		panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
-
-	if (mem_encrypt_active())
-		pr_warn_once("Memory encryption is active and system is using DMA bounce buffers\n");
+#define slot_addr(start, idx)	((start) + ((idx) << IO_TLB_SHIFT))
 

-	if (mapping_size > alloc_size) {
-		dev_warn_once(hwdev, "Invalid sizes (mapping: %zd bytes, alloc: %zd bytes)",
-			      mapping_size, alloc_size);
-		return (phys_addr_t)DMA_MAPPING_ERROR;
-	}
-
-	mask = dma_get_seg_boundary(hwdev);
+/*
+ * Carefully handle integer overflow which can occur when boundary_mask == ~0UL.
+ */
+static inline unsigned long get_max_slots(unsigned long boundary_mask)
+{
+	if (boundary_mask == ~0UL)
+		return 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);
+	return nr_slots(boundary_mask + 1);
+}
 

-	tbl_dma_addr &= mask;
+static unsigned int wrap_index(unsigned int index)
+{
+	if (index >= io_tlb_nslabs)
+		return 0;
+	return index;
+}
 

-	offset_slots = nr_slots(tbl_dma_addr);
+/*
+ * Find a suitable number of IO TLB entries size that will fit this request and
+ * allocate a buffer from that IO TLB pool.
+ */
+static int find_slots(struct device *dev, size_t alloc_size)
+{
+	unsigned long boundary_mask = dma_get_seg_boundary(dev);
+	dma_addr_t tbl_dma_addr =
+		phys_to_dma_unencrypted(dev, io_tlb_start) & boundary_mask;
+	unsigned int max_slots = get_max_slots(boundary_mask);