linux-kernel - Re: [PATCH v5 0/7] arm64: Default to 32-bit wide ZONE

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Message-ID: <20201030181134.GE23196@gaia>
Date:   Fri, 30 Oct 2020 18:11:35 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     robh+dt@...nel.org, hch@....de, ardb@...nel.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org, jeremy.linton@....com,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        will@...nel.org, lorenzo.pieralisi@....com, guohanjun@...wei.com,
        linux-acpi@...r.kernel.org, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v5 0/7] arm64: Default to 32-bit wide ZONE_DMA

On Thu, Oct 29, 2020 at 06:25:43PM +0100, Nicolas Saenz Julienne wrote:
> Ard Biesheuvel (1):
>   arm64: mm: Set ZONE_DMA size based on early IORT scan
> 
> Nicolas Saenz Julienne (6):
>   arm64: mm: Move reserve_crashkernel() into mem_init()
>   arm64: mm: Move zone_dma_bits initialization into zone_sizes_init()
>   of/address: Introduce of_dma_get_max_cpu_address()
>   of: unittest: Add test for of_dma_get_max_cpu_address()
>   arm64: mm: Set ZONE_DMA size based on devicetree's dma-ranges
>   mm: Remove examples from enum zone_type comment

Thanks for putting this together. I had a minor comment but the patches
look fine to me. We still need an ack from Rob on the DT patch and I can
queue the series for 5.11.

Could you please also test the patch below on top of this series? It's
the removal of the implied DMA offset in the max_zone_phys()
calculation.

--------------------------8<-----------------------------
>From 3ae252d888be4984a612236124f5b099e804c745 Mon Sep 17 00:00:00 2001
From: Catalin Marinas <catalin.marinas@....com>
Date: Fri, 30 Oct 2020 18:07:34 +0000
Subject: [PATCH] arm64: Ignore any DMA offsets in the max_zone_phys()
 calculation

Currently, the kernel assumes that if RAM starts above 32-bit (or
zone_bits), there is still a ZONE_DMA/DMA32 at the bottom of the RAM and
such constrained devices have a hardwired DMA offset. In practice, we
haven't noticed any such hardware so let's assume that we can expand
ZONE_DMA32 to the available memory if no RAM below 4GB. Similarly,
ZONE_DMA is expanded to the 4GB limit if no RAM addressable by
zone_bits.

Signed-off-by: Catalin Marinas <catalin.marinas@....com>
---
 arch/arm64/mm/init.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 095540667f0f..362160e16fb2 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -175,14 +175,21 @@ static void __init reserve_elfcorehdr(void)
 #endif /* CONFIG_CRASH_DUMP */
 
 /*
- * 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.
+ * Return the maximum physical address for a zone accessible by the given bits
+ * limit. If the DRAM starts above 32-bit, expand the zone to the maximum
+ * available memory, otherwise cap it at 32-bit.
  */
 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 zone_mask = (1ULL << zone_bits) - 1;
+	phys_addr_t phys_start = memblock_start_of_DRAM();
+
+	if (!(phys_start & U32_MAX))
+		zone_mask = PHYS_ADDR_MAX;
+	else if (!(phys_start & zone_mask))
+		zone_mask = U32_MAX;
+
+	return min(zone_mask + 1, memblock_end_of_DRAM());
 }
 
 static void __init zone_sizes_init(unsigned long min, unsigned long max)