linux-kernel - Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal page allocations

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Date:   Thu, 2 May 2019 15:26:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Nicolin Chen <nicoleotsuka@...il.com>, robin.murphy@....com,
        vdumpa@...dia.com, linux@...linux.org.uk, will.deacon@....com,
        joro@...tes.org, m.szyprowski@...sung.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, tony@...mide.com
Subject: Re: [PATCH v2 RFC/RFT 1/5] ARM: dma-mapping: Add fallback normal
 page allocations

On Tue, Apr 30, 2019 at 04:24:21PM +0100, Catalin Marinas wrote:
> My reading of the arm32 __dma_alloc() is that if the conditions are
> right for the CMA allocator (allows blocking) and there is a default CMA
> area or a per-device one, the call ends up in cma_alloc() without any
> fallback if such allocation fails. Whether this is on purpose, I'm not
> entirely sure. There are a couple of arm32 SoCs which call
> dma_declare_contiguous() or dma_contiguous_reserve_area() and a few DT
> files describing a specific CMA range (e.g. arch/arm/boot/dts/sun5i.dtsi
> with a comment that address must be kept in the lower 256MB).
> 
> If ZONE_DMA is set up correctly so that cma_alloc() is (or can be made)
> interchangeable with alloc_pages(GFP_DMA) from a device DMA capability
> perspective , I think it should be fine to have such fallback.

Indeed.  I missed arm32 being different from everyone else, but we
already addresses that in another thread.  Sorry for misleading
everyone.