[RFC PATCH v7 11/13] dma-buf: system_heap: Limit scatterlist entries to the buffer size

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

The system heap currently allocates each backing page no larger than the
remaining dma-buf length. It can therefore use the complete
compound-page size for every scatterlist entry while keeping the total
length equal to the buffer size.

Shared backing allocations may need to be rounded up to an architecture
shared granule size. A backing allocation can then be larger than the
remaining buffer length. Describing the complete allocation in the
scatterlist would incorrectly expose the rounded tail to scatterlist
consumers as part of the dma-buf.

Track the remaining buffer length while constructing the scatterlist and
limit each entry to the smaller of the compound-page size and the
remaining length. The complete backing allocation remains owned by the
heap and is still released normally.

This does not change behavior with the current allocation policy, but
prepares the heap for shared-granule-sized backing allocations.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 drivers/dma-buf/heaps/system_heap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
index c8959eadc71d..b5b8cdf65f23 100644
--- a/drivers/dma-buf/heaps/system_heap.c
+++ b/drivers/dma-buf/heaps/system_heap.c
@@ -406,6 +406,7 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
 	struct system_heap_buffer *buffer;
 	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
 	unsigned long size_remaining = len;
+	unsigned long sg_remaining = len;
 	unsigned int max_order = orders[0];
 	struct system_heap_priv *priv = dma_heap_get_drvdata(heap);
 	bool cc_shared = priv->cc_shared;
@@ -454,7 +455,11 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap,
 
 	sg = table->sgl;
 	list_for_each_entry_safe(page, tmp_page, &pages, lru) {
-		sg_set_page(sg, page, page_size(page), 0);
+		unsigned long sg_len;
+
+		sg_len = min_t(unsigned long, page_size(page), sg_remaining);
+		sg_set_page(sg, page, sg_len, 0);
+		sg_remaining -= sg_len;
 		sg = sg_next(sg);
 		list_del(&page->lru);
 	}
-- 
2.43.0