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Message-Id: <20200819182434.28196-1-nsaenzjulienne@suse.de>
Date: Wed, 19 Aug 2020 20:24:33 +0200
From: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To: linux-kernel@...r.kernel.org
Cc: f.fainelli@...il.com, hch@....de,
linux-rpi-kernel@...ts.infradead.org,
Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org
Subject: [RFC] arm64: mm: Do not use both DMA zones when 30-bit address space unavailable
There is no benefit in splitting the 32-bit address space into two
distinct DMA zones when the 30-bit address space isn't even available on
a device. If that is the case, default to one big ZONE_DMA spanning the
whole 32-bit address space.
This will help reduce some of the issues we've seen with big crash
kernel allocations.
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
---
Whith this patch, on a 8GB RPi4 the setup looks like this:
DMA [mem 0x0000000000000000-0x000000003fffffff]
DMA32 [mem 0x0000000040000000-0x00000000ffffffff]
Normal [mem 0x0000000100000000-0x00000001ffffffff]
And stock 8GB virtme/qemu:
DMA [mem 0x0000000040000000-0x00000000ffffffff]
DMA32 empty
Normal [mem 0x0000000100000000-0x000000023fffffff]
arch/arm64/mm/init.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index b6881d61b818..857a62611d7a 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -183,13 +183,20 @@ static void __init reserve_elfcorehdr(void)
/*
* Return the maximum physical address for a zone with a given address size
- * limit. It currently assumes that for memory starting above 4G, 32-bit
- * devices will use a DMA offset.
+ * limit or zero if memory starts from an address higher than the zone targeted.
+ * It currently assumes that for memory starting above 4G, 32-bit devices will
+ * use a DMA offset.
*/
static phys_addr_t __init max_zone_phys(unsigned int zone_bits)
{
- phys_addr_t offset = memblock_start_of_DRAM() & GENMASK_ULL(63, zone_bits);
- return min(offset + (1ULL << zone_bits), memblock_end_of_DRAM());
+ phys_addr_t base = memblock_start_of_DRAM();
+ phys_addr_t offset = base & GENMASK_ULL(63, 32);
+ s64 zone_size = (1ULL << zone_bits) - (base & DMA_BIT_MASK(32));
+
+ if (zone_size <= 0)
+ return 0;
+
+ return min(base + zone_size + offset, memblock_end_of_DRAM());
}
static void __init zone_sizes_init(unsigned long min, unsigned long max)
@@ -390,6 +397,20 @@ void __init arm64_memblock_init(void)
if (IS_ENABLED(CONFIG_ZONE_DMA)) {
zone_dma_bits = ARM64_ZONE_DMA_BITS;
arm64_dma_phys_limit = max_zone_phys(ARM64_ZONE_DMA_BITS);
+
+ /*
+ * We don't want to split the 32 bit address space into two DMA
+ * zones when the target lower physical address range doesn't
+ * exist. For example, if memory starts at 0x80000000 it's
+ * pointless to create a ZONE_DMA distinct of ZONE_DMA32 as
+ * there's no way to access the 30-bit address space. If that's
+ * the case just expand ZONE_DMA to cover the whole 32-bit
+ * address space.
+ */
+ if (!arm64_dma_phys_limit) {
+ zone_dma_bits = 32;
+ arm64_dma_phys_limit = max_zone_phys(32);
+ }
}
if (IS_ENABLED(CONFIG_ZONE_DMA32))
--
2.28.0
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