Re: [RFC/RFT PATCH 1/2] dma-contiguous: Simplify dma_*_from_contiguous() function calls

[Christoph Hellwig]

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..