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

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@arm.com>
Suggested-by: Ross Burton <ross.burton@arm.com>
Cc: Ross Burton <ross.burton@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

arch/arm64/mm/init.c

@@ -16,6 +16,7 @@
 #include <linux/nodemask.h>
 #include <linux/initrd.h>
 #include <linux/gfp.h>
+#include <linux/math.h>
 #include <linux/memblock.h>
 #include <linux/sort.h>
 #include <linux/of.h>
@@ -493,8 +494,16 @@ 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 =
+			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
+		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
 		swiotlb = true;
+	}
 
 	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);