swiotlb: add comment where we handle the overflow of a dma mask on 32 bit

Impact: cleanup

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index db724ba7ebf6..1272b23e4769 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -301,6 +301,10 @@ map_single(struct device *hwdev, char *buffer, size_t size, int dir)
         start_dma_addr = virt_to_bus(io_tlb_start) & mask;
 
         offset_slots = ALIGN(start_dma_addr, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT;
+
+        /*
+         * Carefully handle integer overflow which can occur when mask == ~0UL.
+         */
         max_slots = mask + 1
                     ? ALIGN(mask + 1, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT
                     : 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);