linux-kernel - Re: [PATCH v6 2/2] arm64: mm: reserve per-numa CMA to localize coherent dma buffers

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Date:   Fri, 21 Aug 2020 10:01:16 +0100
From:   Will Deacon <will@...nel.org>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        ganapatrao.kulkarni@...ium.com, catalin.marinas@....com,
        iommu@...ts.linux-foundation.org, linuxarm@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        huangdaode@...wei.com,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Steve Capper <steve.capper@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v6 2/2] arm64: mm: reserve per-numa CMA to localize
 coherent dma buffers

On Fri, Aug 21, 2020 at 02:26:15PM +1200, Barry Song wrote:
> Right now, smmu is using dma_alloc_coherent() to get memory to save queues
> and tables. Typically, on ARM64 server, there is a default CMA located at
> node0, which could be far away from node2, node3 etc.
> with this patch, smmu will get memory from local numa node to save command
> queues and page tables. that means dma_unmap latency will be shrunk much.
> Meanwhile, when iommu.passthrough is on, device drivers which call dma_
> alloc_coherent() will also get local memory and avoid the travel between
> numa nodes.
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> Cc: Steve Capper <steve.capper@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Mike Rapoport <rppt@...ux.ibm.com>
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
>  -v6: rebase on top of 5.9-rc1
> 
>  arch/arm64/mm/init.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 481d22c32a2e..f1c75957ff3c 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -429,6 +429,8 @@ void __init bootmem_init(void)
>  	arm64_hugetlb_cma_reserve();
>  #endif
>  
> +	dma_pernuma_cma_reserve();

I think will have to do for now, but I still wish that more of this was
driven from the core code so that we don't have to worry about
initialisation order and whether things are early/late enough on a per-arch
basis.

Acked-by: Will Deacon <will@...nel.org>

Will