linux-kernel - Re: [RFC/RFT PATCH 1/2] dma-contiguous: Simplify dma_*_from

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Date:   Tue, 30 Apr 2019 12:56:40 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     hch@....de, robin.murphy@....com, m.szyprowski@...sung.com,
        vdumpa@...dia.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, chris@...kel.net, jcmvbkbc@...il.com,
        joro@...tes.org, dwmw2@...radead.org, tony@...mide.com,
        akpm@...ux-foundation.org, sfr@...b.auug.org.au,
        treding@...dia.com, keescook@...omium.org, iamjoonsoo.kim@....com,
        wsa+renesas@...g-engineering.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, iommu@...ts.linux-foundation.org
Subject: Re: [RFC/RFT PATCH 1/2] dma-contiguous: Simplify
 dma_*_from_contiguous() function calls

So while I really, really like this cleanup it turns out it isn't
actually safe for arm :(  arm remaps the CMA allocation in place
instead of using a new mapping, which can be done because they don't
share PMDs with the kernel.

So we'll probably need a __dma_alloc_from_contiguous version with
an additional bool fallback argument - everyone but arms uses
dma_alloc_from_contiguous as in your patch, just arm will get the
non-fallback one.

Sorry for not sorting this our earlier.