[PATCH] dma-buf: system_heap: align allocation orders to ARM64 hugepage sizes

[Pankaj Patil]

From: Prakash Gupta <prakash.gupta@xxxxxxxxxxxxxxxx>

System heap currently uses a fixed allocation order, {8, 4, 0} while
requesting pages from the buddy allocator which correspond to 1 MiB,
64 KiB and 4 KiB only on systems using 4 KiB page size.

Extending on patch from Juan [1] for ARM64 targets with LPAE, a 1 MiB
allocation does not correspond to a PMD block size. For a 4 KiB-granule
IOMMU, a 2 MiB allocation can be mapped using a single PMD block entry,
reducing IOMMU TLB pressure by requiring fewer translation entries.

Use allocation orders corresponding to the ARM64 PMD block and
contiguous-PTE sizes for 4 KiB, 16 KiB and 64 KiB page configurations.
ARM64 applicable orders are selected for ARM64 targets with default
orders being used for rest of the architectures.

[1] https://lore.kernel.org/all/20250417201913.2393655-1-jyescas@xxxxxxxxxx/

Co-developed-by: Juan Yescas <jyescas@xxxxxxxxxx>
Signed-off-by: Juan Yescas <jyescas@xxxxxxxxxx>
Co-developed-by: Pankaj Patil <pankaj.patil@xxxxxxxxxxxxxxxx>
Signed-off-by: Pankaj Patil <pankaj.patil@xxxxxxxxxxxxxxxx>
Signed-off-by: Prakash Gupta <prakash.gupta@xxxxxxxxxxxxxxxx>
---
 drivers/dma-buf/heaps/system_heap.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
index c8959eadc71d..7a3e26c3dd43 100644
--- a/drivers/dma-buf/heaps/system_heap.c
+++ b/drivers/dma-buf/heaps/system_heap.c
@@ -22,6 +22,7 @@
 #include <linux/module.h>
 #include <linux/pgtable.h>
 #include <linux/scatterlist.h>
+#include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 
@@ -57,12 +58,30 @@ struct dma_heap_attachment {
 				| __GFP_COMP)
 static gfp_t order_flags[] = {HIGH_ORDER_GFP, HIGH_ORDER_GFP, LOW_ORDER_GFP};
 /*
- * The selection of the orders used for allocation (1MB, 64K, 4K) is designed
- * to match with the sizes often found in IOMMUs. Using order 4 pages instead
- * of order 0 pages can significantly improve the performance of many IOMMUs
- * by reducing TLB pressure and time spent updating page tables.
+ * For ARM64, allocation orders correspond to the PMD block and CONT PTE
+ * hugepage sizes for each CPU page granule, which also align with the
+ * TLB entry sizes of a 4K-granule IOMMU (arm-smmu, arm-smmu-v3).
+ * Each order = (level_shift - PAGE_SHIFT), derived from:
+ *   PMD_SHIFT      = (PAGE_SHIFT - 3) * 2 + 3
+ *   CONT_PTE_SHIFT = CONFIG_ARM64_CONT_PTE_SHIFT + PAGE_SHIFT
+ *
+ * 4K  CPU (PAGE_SHIFT=12): PMD=2MiB   (21-12=9),  CONT_PTE=64KiB  (16-12=4)
+ * 16K CPU (PAGE_SHIFT=14): PMD=32MiB  (25-14=11), CONT_PTE=2MiB   (21-14=7)
+ * 64K CPU (PAGE_SHIFT=16): PMD=512MiB (29-16=13), CONT_PTE=2MiB   (21-16=5)
+ */
+#if defined(CONFIG_ARM64) && (PAGE_SIZE == SZ_4K)
+static const unsigned int orders[] = {21 - PAGE_SHIFT, 16 - PAGE_SHIFT, 0};
+#elif defined(CONFIG_ARM64) && (PAGE_SIZE == SZ_16K)
+static const unsigned int orders[] = {25 - PAGE_SHIFT, 21 - PAGE_SHIFT, 0};
+#elif defined(CONFIG_ARM64) && (PAGE_SIZE == SZ_64K)
+static const unsigned int orders[] = {29 - PAGE_SHIFT, 21 - PAGE_SHIFT, 0};
+#else
+/* Fallback to default allocation order for non-arm64 architectures
+ * On 4K-page systems, these orders correspond to 1MiB, 64KiB and 4KiB,
+ * matching sizes commonly supported by IOMMUs.
  */
 static const unsigned int orders[] = {8, 4, 0};
+#endif
 #define NUM_ORDERS ARRAY_SIZE(orders)
 
 static int system_heap_set_page_decrypted(struct page *page)

---
base-commit: c2cd463d6ee7d55a3ec0719d93c49ff99022d58f
change-id: 20260916-dma_buf_improvement-a095ae567e5f

Best regards,
-- 
Pankaj Patil <pankaj.patil@xxxxxxxxxxxxxxxx>