[PATCH] arm64: fix CONFIG_ZONE_DMA on systems with no 32-bit addressable DRAM

[msalter@redhat.com (Mark Salter)]

Commit 2d5a5612bc (arm64: Limit the CMA buffer to 32-bit if ZONE_DMA)
forces the CMA buffer to be 32-bit addressable if CONFIG_ZONE_DMA is
defined. This breaks CMA on platforms with no 32-bit addressable DRAM.
This patch checks to make sure there is 32-bit addressable DRAM before
setting the 32-bit limit. If there is none, no limit is placed on the
CMA buffer. This allows a single kernel (with CONFIG_ZONE_DMA defined)
to support platforms requiring the 32-bit limit and platforms with no
32-bit limit.

Signed-off-by: Mark Salter <msalter@redhat.com>
---
 arch/arm64/mm/init.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index c5415e2..2925576 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -149,8 +149,17 @@ void __init arm64_memblock_init(void)
 	early_init_fdt_scan_reserved_mem();
 
 	/* 4GB maximum for 32-bit only capable devices */
-	if (IS_ENABLED(CONFIG_ZONE_DMA))
+	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
 		dma_phys_limit = dma_to_phys(NULL, DMA_BIT_MASK(32)) + 1;
+		/*
+		 * If platform doesn't have DRAM within the dma_phys_limit,
+		 * remove the limit altogether. This allows one kernel (with
+		 * CONFIG_ZONE_DMA defined) to support platforms with 32-bit
+		 * only devices and platforms with no 32-bit DRAM.
+		 */
+		if (dma_phys_limit <= memblock_start_of_DRAM())
+			dma_phys_limit = 0;
+	}
 	dma_contiguous_reserve(dma_phys_limit);
 
 	memblock_allow_resize();
-- 
1.8.3.1