linux-kernel - [PATCH 3/4] arm64: Default to 32-bit ZONE

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Date:   Thu,  1 Oct 2020 18:17:39 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     robh+dt@...nel.org, catalin.marinas@....com, hch@....de,
        robin.murphy@....com, devicetree@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Subject: [PATCH 3/4] arm64: Default to 32-bit ZONE_DMA

The Raspberry Pi 4 needs two DMA zones as some of its devices can only
DMA into the 30-bit physical address space. We solved that by creating
an extra ZONE_DMA covering the 30-bit. It turns out that creating extra
zones unnecessarily broke Kdump on large systems. So default to a single
32-bit wide ZONE_DMA and only define both zones if running on RPi4.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
---
 arch/arm64/mm/init.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index e1a69a618832..3c3f462466eb 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -43,8 +43,6 @@
 #include <asm/tlb.h>
 #include <asm/alternative.h>
 
-#define ARM64_ZONE_DMA_BITS	30
-
 /*
  * We need to be able to catch inadvertent references to memstart_addr
  * that occur (potentially in generic code) before arm64_memblock_init()
@@ -388,8 +386,14 @@ void __init arm64_memblock_init(void)
 	early_init_fdt_scan_reserved_mem();
 
 	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
-		zone_dma_bits = ARM64_ZONE_DMA_BITS;
-		arm64_dma_phys_limit = max_zone_phys(ARM64_ZONE_DMA_BITS);
+		/*
+		 * early_init_dt_scan() might alter zone_dma_bits based on the
+		 * device's DT. Otherwise, have it cover the 32-bit address
+		 * space.
+		 */
+		if (zone_dma_bits == ZONE_DMA_BITS_DEFAULT)
+			zone_dma_bits = 32;
+		arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
 	}
 
 	if (IS_ENABLED(CONFIG_ZONE_DMA32))
-- 
2.28.0