[PATCH] arm64: swiotlb: Reduce the default size if no ZONE_DMA bouncing needed

[Catalin Marinas]

With CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC enabled, the arm64 kernel still
allocates the default SWIOTLB buffer (64MB) even if ZONE_DMA is disabled
or all the RAM fits into this zone. However, this potentially wastes a
non-negligible amount of memory on platforms with little RAM.

Reduce the SWIOTLB size to 1MB per 1GB of RAM if only needed for
kmalloc() buffer bouncing.

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Suggested-by: Ross Burton <ross.burton at arm.com>
Cc: Ross Burton <ross.burton at arm.com>
Cc: Robin Murphy <robin.murphy at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/mm/init.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 8a0f8604348b..54ee1a4868c2 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -493,8 +493,16 @@ void __init mem_init(void)
 {
 	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
 
-	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC))
+	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
+		/*
+		 * If no bouncing needed for ZONE_DMA, reduce the swiotlb
+		 * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.
+		 */
+		unsigned long size =
+			ALIGN(memblock_phys_mem_size(), SZ_1G) >> 10;
+		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
 		swiotlb = true;
+	}
 
 	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);