Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

Leon Romanovsky
2024-07-04T13:18:44Z

On Thu, Jul 04, 2024 at 09:48:56AM +0200, Christoph Hellwig wrote:
> On Wed, Jul 03, 2024 at 06:51:14PM +0300, Leon Romanovsky wrote:
> > If we put aside this issue, do you think that the proposed API is the right one?
>
> I haven't look at it in detail yet, but from a quick look there is a
> few things to note:
>
>
> 1) The amount of code needed in nvme worries me a bit. Now NVMe a messy
> driver due to the stupid PRPs vs just using SGLs, but needing a fair
> amount of extra boilerplate code in drivers is a bit of a warning sign.
> I plan to look into this to see if I can help on improving it, but for
> that I need a working version first.

Chaitanya is working on this and I'll join him to help on next Sunday,
after I'll return to the office from my sick leave/

>
>
> 2) The amount of seemingly unrelated global headers pulled into other
> global headers. Some of this might just be sloppiness, e.g. I can't
> see why dma-mapping.h would actually need iommu.h to start with,
> but pci.h in dma-map-ops.h is a no-go.

pci.h was pulled because I needed to call to pci_p2pdma_map_type()
in dma_can_use_iova().

>
> 3) which brings me to real layering violations. dev_is_untrusted and
> dev_use_swiotlb are DMA API internals, no way I'd ever want to expose
> them. dma-map-ops.h is a semi-internal header only for implementations
> of the dma ops (as very clearly documented at the top of that file),
> it must not be included by drivers. Same for swiotlb.h.

These item shouldn't worry you and will be changed in the final version.
They are outcome of patch "RDMA/umem: Prevent UMEM ODP creation with SWIOTLB".
https://lore.kernel.org/all/d18c454636bf3cfdba9b66b7cc794d713eadc4a5.1719909395.git.leon@kernel.org/

All HMM users need such "prevention" so it will be moved to a common place.

>
> Not quite as concerning, but doing an indirect call for each map
> through dma_map_ops in addition to the iommu ops is not every efficient.
> We've through for a while to allow direct calls to dma-iommu similar
> how we do direct calls to dma-direct from the core mapping.c code.
> This might be a good time to do that as a prep step for this work.

Sure, no problem, will start in parallel to work on this.

>