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

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Date:   Tue, 30 Apr 2019 13:37:54 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>,
        Nicolin Chen <nicoleotsuka@...il.com>
Cc:     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

On 30/04/2019 11:56, Christoph Hellwig wrote:
> 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.

Or we even just implement dma_{alloc,free}_contiguous() as a wrapper 
around the existing APIs so that users can be thoroughly checked and 
converted one-by-one.

Robin.