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

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Message-ID: <20190430151833.GB25447@lst.de>
Date:   Tue, 30 Apr 2019 17:18:33 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Nicolin Chen <nicoleotsuka@...il.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

On Tue, Apr 30, 2019 at 01:37:54PM +0100, Robin Murphy wrote:
> 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.

Yeah.  Actually given all the contention I wonder if the easiest solution
for now is to just open code the cma_alloc/cma_free calls in dma-direct
and dma-iommu, with the hopes that everyone is going to migrate to those
implementations in the mid-term anyway and dma_alloc_from_contiguous /
dma_release_from_contiguous just go away..