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

[Nicolin Chen]

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