Messages in this thread | | | Date | Fri, 7 Apr 2023 07:55:48 +0200 | From | Christoph Hellwig <> | Subject | Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce buffers |
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On Tue, Mar 28, 2023 at 09:54:35AM +0200, Petr Tesarik wrote: > I tend to agree here. However, it's the DMABUF design itself that causes > some trouble. The buffer is allocated by the v3d driver, which does not > have the restriction, so the DMA API typically allocates an address > somewhere near the 4G boundary. Userspace then exports the buffer, sends > it to another process as a file descriptor and imports it into the vc4 > driver, which requires DMA below 1G. In the beginning, v3d had no idea > that the buffer would be exported to userspace, much less that it would > be later imported into vc4.
Then we need to either:
a) figure out a way to communicate these addressing limitations b) find a way to migrate a buffer into other memory, similar to how page migration works for page cache
> BTW my testing also suggests that the streaming DMA API is quite > inefficient, because UAS performance _improved_ with swiotlb=force. > Sure, this should probably be addressed in the UAS and/or xHCI driver, > but what I mean is that moving away from swiotlb may even cause > performance regressions, which is counter-intuitive. At least I would > _not_ have expected it.
That is indeed very odd. Are you running with a very slow iommu driver there? Or what is the actual use case there in general?
> >> + gfp = (attrs & DMA_ATTR_MAY_SLEEP) ? GFP_KERNEL : GFP_NOWAIT; > >> + slot = kmalloc(sizeof(*slot), gfp | __GFP_NOWARN); > >> + if (!slot) > >> + goto err; > >> + > >> + slot->orig_addr = orig_addr; > >> + slot->alloc_size = alloc_size; > >> + slot->page = dma_direct_alloc_pages(dev, PAGE_ALIGN(alloc_size), > >> + &slot->dma_addr, dir, > >> + gfp | __GFP_NOWARN); > >> + if (!slot->page) > >> + goto err_free_slot; > > > > Without GFP_NOIO allocations this will deadlock eventually. > > Ah, that would affect the non-sleeping case (GFP_KERNEL), right?
Yes.
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