linux-kernel - Re: [PATCH v6 RESED 2/2] arm64: support DMA zone above 4GB

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Message-ID: <20240812075452.4ef3eef5@mordecai.tesarici.cz>
Date: Mon, 12 Aug 2024 07:54:52 +0200
From: Petr Tesarik <ptesarik@...e.com>
To: Baruch Siach <baruch@...s.co.il>
Cc: Christoph Hellwig <hch@....de>, Marek Szyprowski
 <m.szyprowski@...sung.com>, Catalin Marinas <catalin.marinas@....com>, Will
 Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org, Ramon Fried <ramon@...reality.ai>, Elad Nachman
 <enachman@...vell.com>, Petr Tesařík <petr@...arici.cz>
Subject: Re: [PATCH v6 RESED 2/2] arm64: support DMA zone above 4GB

On Sun, 11 Aug 2024 10:09:36 +0300
Baruch Siach <baruch@...s.co.il> wrote:

> From: Catalin Marinas <catalin.marinas@....com>
> 
> Commit 791ab8b2e3db ("arm64: Ignore any DMA offsets in the
> max_zone_phys() calculation") made arm64 DMA/DMA32 zones span the entire
> RAM when RAM starts above 32-bits. This breaks hardware with DMA area
> that start above 32-bits. But the commit log says that "we haven't
> noticed any such hardware". It turns out that such hardware does exist.
> 
> One such platform has RAM starting at 32GB with an internal bus that has
> the following DMA limits:
> 
>   #address-cells = <2>;
>   #size-cells = <2>;
>   dma-ranges = <0x00 0xc0000000 0x08 0x00000000 0x00 0x40000000>;
> 
> That is, devices under this bus see 1GB of DMA range between 3GB-4GB in
> their address space. This range is mapped to CPU memory at 32GB-33GB.
> With current code DMA allocations for devices under this bus are not
> limited to DMA area, leading to run-time allocation failure.
> 
> This commit reinstates DMA zone at the bottom of RAM. The result is DMA
> zone that properly reflects the hardware constraints as follows:
> 
> [    0.000000] Zone ranges:
> [    0.000000]   DMA      [mem 0x0000000800000000-0x000000083fffffff]
> [    0.000000]   DMA32    empty
> [    0.000000]   Normal   [mem 0x0000000840000000-0x0000000bffffffff]
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> [baruch: split off the original patch]
> Signed-off-by: Baruch Siach <baruch@...s.co.il>

Note that I'm not an Arm64 maintainer, so the value of my review is
limited, but AFAICS this change should work as intended.

Reviewed-by: Petr Tesarik <ptesarik@...e.com>

Petr T

> ---
>  arch/arm64/mm/init.c | 12 ------------
>  1 file changed, 12 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index c45e2152ca9e..bfb10969cbf0 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -114,20 +114,8 @@ static void __init arch_reserve_crashkernel(void)
>  				    low_size, high);
>  }
>  
> -/*
> - * Return the maximum physical address for a zone given its limit.
> - * If 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(phys_addr_t zone_limit)
>  {
> -	phys_addr_t phys_start = memblock_start_of_DRAM();
> -
> -	if (phys_start > U32_MAX)
> -		zone_limit = PHYS_ADDR_MAX;
> -	else if (phys_start > zone_limit)
> -		zone_limit = U32_MAX;
> -
>  	return min(zone_limit, memblock_end_of_DRAM() - 1) + 1;
>  }
>