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

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Date:   Tue, 30 Apr 2019 12:46:13 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Robin Murphy <robin.murphy@....com>, Christoph Hellwig <hch@....de>
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 Tue, Apr 30, 2019 at 05:18:33PM +0200, Christoph Hellwig wrote:
> 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..

Thanks for the comments.

Listing all the solutions as a summary:
A) Add "bool fallback" to dma_{alloc,free}_contiguous, and let
   ARM use fallback=false.
B) Continue replacing "_from" with dma_{alloc,free}_contiguous
   but let callers like ARM use cma_alloc/free() directly.
C) Have both new dma_{alloc,free}_contiguous and "_from" funcs.
   Implement the new one to dma-direct only as an initial step
   and change others one-by-one in the future.

Combining the comments at alloc_pages_node(), I guess that the
Solution C would be a better (cleaner) one?

List of to-change callers for Solution C:
 kernel/dma/direct.c

List of to-exclude-for-now callers for Solution C:
 arch/arm64/mm/dma-mapping.c
 drivers/iommu/amd_iommu.c
 drivers/iommu/intel-iommu.c
 arch/arm/mm/dma-mapping.c
 arch/xtensa/kernel/pci-dma.c
 kernel/dma/remap.c