Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

[Christoph Hellwig]

On Thu, Sep 28, 2023 at 01:10:27AM +0200, Linus Walleij wrote:
> (...)
> > +       select DMA_DIRECT_REMAP
> 
> Christoph invented that symbol so he can certainly
> explain what is missing to use this on ARM.
> 
> This looks weird to me, because:
> > git grep atomic_pool_init
> arch/arm/mm/dma-mapping.c:static int __init atomic_pool_init(void)
> kernel/dma/pool.c:static int __init dma_atomic_pool_init(void)
> 
> Now you have two atomic DMA pools in the kernel,
> and a lot more than that is duplicated. I'm amazed that it
> compiles at all.
> 
> Clearly if you want to do this, surely the ARM-specific
> arch/arm/mm/dma-mapping.c and arch/arm/mm/dma-mapping-nommu.c
> needs to be removed at the same time?
> 
> However I don't think it's that simple, because Christoph would surely
> had done this a long time ago if it was that simple.

Yes, DMA_DIRECT_REMAP should only be used for platforms using the
generic generic remap that plus straight into dma-direct and
bypasses arch_dma_alloc.

ARM first needs support to directly set the uncached/wc bits on
the direct mapping for CMA, which should be fairly simple but require
wide spread testing.

I'd be happy to work with anyone who wants to look into this.