[PATCH] dma-buf: system_heap: Optimize sg_table-to-pages conversion in vmap

["Barry Song (Xiaomi)" <baohua@kernel.org>]

From: Xueyuan Chen <Xueyuan.chen21@gmail.com>

Replace the heavy for_each_sgtable_page() iterator in system_heap_do_vmap()
with a more efficient nested loop approach.

Instead of iterating page by page, we now iterate through the scatterlist
entries via for_each_sgtable_sg(). Because pages within a single sg entry
are physically contiguous, we can populate the page array with a in an
inner loop using simple pointer math. This save a lot of time.

The WARN_ON check is also pulled out of the loop to save branch
instructions.

Performance results mapping a 2GB buffer on Radxa O6:
- Before: ~1440000 ns
- After:  ~232000 ns
(~84% reduction in iteration time, or ~6.2x faster)

Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Cc: Brian Starkey <Brian.Starkey@arm.com>
Cc: John Stultz <jstultz@google.com>
Cc: T.J. Mercier <tjmercier@google.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Xueyuan Chen <Xueyuan.chen21@gmail.com>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 drivers/dma-buf/heaps/system_heap.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
index b3650d8fd651..769f01f0cc96 100644
--- a/drivers/dma-buf/heaps/system_heap.c
+++ b/drivers/dma-buf/heaps/system_heap.c
@@ -224,16 +224,21 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer)
 	int npages = PAGE_ALIGN(buffer->len) / PAGE_SIZE;
 	struct page **pages = vmalloc(sizeof(struct page *) * npages);
 	struct page **tmp = pages;
-	struct sg_page_iter piter;
 	void *vaddr;
+	u32 i, j, count;
+	struct page *base_page;
+	struct scatterlist *sg;
 
 	if (!pages)
 		return ERR_PTR(-ENOMEM);
 
-	for_each_sgtable_page(table, &piter, 0) {
-		WARN_ON(tmp - pages >= npages);
-		*tmp++ = sg_page_iter_page(&piter);
+	for_each_sgtable_sg(table, sg, i) {
+		base_page = sg_page(sg);
+		count = sg->length >> PAGE_SHIFT;
+		for (j = 0; j < count; j++)
+			*tmp++ = base_page + j;
 	}
+	WARN_ON(tmp - pages != npages);
 
 	vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL);
 	vfree(pages);
-- 
2.39.3 (Apple Git-146)