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

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Date:   Wed, 24 Apr 2019 21:26:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Christoph Hellwig <hch@....de>, robin.murphy@....com,
        vdumpa@...dia.com, linux@...linux.org.uk, catalin.marinas@....com,
        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 Wed, Apr 24, 2019 at 11:33:11AM -0700, Nicolin Chen wrote:
> I feel it's similar to my previous set, which did most of these
> internally except the renaming part. But Catalin had a concern
> that some platforms might have limits on CMA range [1]. Will it
> be still okay to do the fallback internally?
> 
> [1: https://www.spinics.net/lists/arm-kernel/msg714295.html ]

Catalins statement is correct, but I don't see how it applies to
your patch.  Your patch just ensures that the fallback we have
in most callers is uniformly applied everywhere.  The non-iommu
callers will still need to select a specific zone and/or retry
just the page allocator with other flags if the CMA (or fallback)
page doesn't match what they need.  dma-direct does this correctly
and I think the arm32 allocator does as well, although it is a bit
hard to follow sometimes.