Re: [PATCH 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP

From: Christoph Hellwig
Date: Mon Feb 11 2019 - 11:39:41 EST

On Wed, Feb 06, 2019 at 11:55:49AM +0000, Robin Murphy wrote:
> On 14/01/2019 09:41, Christoph Hellwig wrote:
>> For entirely dma coherent architectures there is no good reason to ever
>> remap dma coherent allocation.
> Yes there is, namely assembling large buffers without the need for massive
> CMA areas and compaction overhead under memory fragmentation. That has
> always been a distinct concern from the DMA_DIRECT_REMAP cases; they've
> just been able to share a fair few code paths.

Well, I guess I need to reword this - there is no _requirement_ to
remap. And x86 has been happy to not remap so far and I see absolutely
no reason to force anyone to remap.

>> Move all the remap and pool code under
>> CONFIG_DMA_DIRECT_REMAP ifdefs, and drop the Kconfig dependency.
> As far as I'm concerned that splits things the wrong way. Logically,
> iommu_dma_alloc() should always have done its own vmap() instead of just
> returning the bare pages array, but that was tricky to resolve with the
> design of having the caller handle everything to do with coherency (forcing
> the caller to unpick that mapping just to remap it yet again in the
> noncoherent case didn't seem sensible).

I don't parse this. In the old code base before this series
iommu_dma_alloc is a relatively low-level helper allocating and mapping
pages. And that one should have done the remapping, and in fact does
so since ("dma-iommu: refactor page array remap helpers"). It just
happens that the function is now called iommu_dma_alloc_remap.

The new iommu_dma_alloc is the high level entry point that handles
every possible case of different allocations, including those where
we do not have a virtual mapping.