Messages in this thread | | | From | Laurentiu Tudor <> | Subject | Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys_addr() | Date | Thu, 24 Oct 2019 11:27:17 +0000 |
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On 24.10.2019 14:04, Robin Murphy wrote: > On 2019-10-24 8:49 am, Laurentiu Tudor wrote: >> >> >> On 24.10.2019 05:01, hch@lst.de wrote: >>> On Wed, Oct 23, 2019 at 11:53:41AM +0000, Laurentiu Tudor wrote: >>>> We had an internal discussion over these points you are raising and >>>> Madalin (cc-ed) came up with another idea: instead of adding this prone >>>> to misuse api how about experimenting with a new dma unmap and dma sync >>>> variants that would return the physical address by calling the newly >>>> introduced dma map op. Something along these lines: >>>> * phys_addr_t dma_unmap_page_ret_phys(...) >>>> * phys_addr_t dma_unmap_single_ret_phys(...) >>>> * phys_addr_t dma_sync_single_for_cpu_ret_phys(...) >>>> I'm thinking that this proposal should reduce the risks opened by the >>>> initial variant. >>>> Please let me know what you think. >>> >>> I'm not sure what the ret is supposed to mean, but I generally like >>> that idea better. >> >> It was supposed to be short for "return" but given that I'm not good at >> naming stuff I'll just drop it. > > Hmm, how about something like "dma_unmap_*_desc" for the context of the > mapped DMA address also being used as a descriptor token?
Alright.
>>> We also need to make sure there is an easy way >>> to figure out if these APIs are available, as they generally aren't >>> for any non-IOMMU API IOMMU drivers. >> >> I was really hoping to manage making them as generic as possible but >> anyway, I'll start working on a PoC and see how it turns out. This will >> probably happen sometime next next week as the following week I'll be >> traveling to a conference. > > AFAICS, even a full implementation of these APIs would have to be > capable of returning an indication that there is no valid physical > address - e.g. if unmap is called with a bogus DMA address that was > never mapped. At that point there'sseemingly no problem just > implementing the trivial case on top of any existing unmap/sync > callbacks for everyone. I'd imagine that drivers which want this aren't > likely to run on the older architectures where the weird IOMMUs live, so > they could probably just always treat failure as unexpected and fatal > either way. > > In fact, I'm now wondering whether it's likely to be common that users > want the physical address specifically, or whether it would make sense > to return the original VA/page, both for symmetry with the corresponding > map calls and for the ease of being able to return NULL when necessary.
That's sounds wonderful as it should make the code leaner in the drivers.
--- Best Regards, Laurentiu | |