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

Leon Romanovsky
2024-07-07T09:45:51Z

On Fri, Jul 05, 2024 at 08:39:10AM +0200, Christoph Hellwig wrote:
> Review from the NVMe driver consumer perspective. I think if all these
> were implement we'd probably end up with less code than before the
> conversion.

Thanks for the review, I will try to address all the comments in the next version.

>
> The split between dma_iova_attrs, dma_memory_type and dma_iova_state is
> odd. I would have expected them to just be just a single object. While
> talking about this I think the domain field in dma_iova_state should
> probably be a private pointer instead of being tied to the iommu.
>
> Also do we need the attrs member in the iova_attrs structure? The
> "attrs" really are flags passed to the mapping routines that are
> per-operation and not persistent, so I'd expect them to be passed
> per-call and not stored in a structure.

It is left-over from my not-send version where I added new attribute
to indicate that dma_alloc_iova() can't support SWIOTLB to avoid
dev_use_swiotlb() mess. I will remove it.

>
> I'd also expect that the use_iova field to be in the mapping state
> and not separately provided by the driver.
>
> For nvme specific data structures I would have expected a dma_add/
> len pair in struct iod_dma_map, maybe even using a common type.
>
> Also the data structure split seems odd - I'd expect the actual
> mapping state and a small number (at least one) dma_addr/len pair
> to be inside the nvme_iod structure, and then only do the dynamic
> allocation if we need more of them because there are more segments
> and we are not using the iommu.
>
> If we had a common data structure for the dma_addr/len pairs
> dma_unlink_range could just take care of the unmap for the non-iommu
> case as well, which would be neat. I'd also expect that
> dma_free_iova would be covered by it.

Internally Jason asked for the same thing, but I didn't want to produce
asymmetric API where drivers have a call to dma_alloc_iova() but don't
have a call to dma_free_iova(). However, now, it is 2 versus 1, so I will
change it.

>
> I would have expected dma_link_range to return the dma_addr_t instead
> of poking into the iova structure in the callers.
>
> In __nvme_rq_dma_map the <= PAGE_SIZE case is pointless. In the
> existing code the reason for it is to avoid allocating and mapping the
> sg_table, but that code is still left before we even get to this code.
>
> My suggestion above to only allocate the dma_addr/len pairs when there
> is more than 1 or a few of it would allow to trivially implement that
> suggestion using the normal API without having to keep that special
> case and the dma_len parameter around.
>
> If this addes a version of dma_map_page_atttrs that directly took
> the physical address as a prep patch the callers would not have to
> bother with page pointer manipulations and just work on physical
> addresses for both the iommu and no-iommu cases. It would also help
> a little bit with the eventualy switch to store the physical address
> instead of page+offset in the bio_vec. Talking about that, I've
> been wanting to add a bvec_phys helper for to convert the
> page_phys(bv.bv_page) + bv.bv_offset calculations. This is becoming
> more urgent with more callers needing to that, I'll try to get it out
> to Jens ASAP so that it can make the 6.11 merge window.
>
> Can we make dma_start_range / dma_end_range simple no-ops for the
> non-iommu code to avoid boilerplate code in the callers to avoid
> boilerplate code in the callers to deal with the two cases?

Yes, sure.

Thanks