On Fri, Apr 20, 2018 at 05:21:11PM +0200, Daniel Vetter wrote: > > At the very lowest level they will need to be handled differently for > > many architectures, the questions is at what point we'll do the > > branching out. > > Having at least struct page also in that list with (dma_addr_t, lenght) > pairs has a bunch of benefits for drivers in unifying buffer handling > code. You just pass that one single list around, use the dma_addr_t side > for gpu access (generally bashing it into gpu ptes). And the struct page > (if present) for cpu access, using kmap or vm_insert_*. We generally > ignore virt, if we do need a full mapping then we construct a vmap for > that buffer of our own. Well, for mapping a resource (which gets back to the start of the discussion) you will need an explicit virt pointer. You also need an explicit virt pointer and not just page_address/kmap for users of dma_get_sgtable, because for many architectures you will need to flush the virtual address used to access the data, which might be a vmap/ioremap style mapping retourned from dma_alloc_address, and not the directly mapped kernel address. Here is another idea at the low-level dma API level: - dma_get_sgtable goes away. The replacement is a new dma_alloc_remap helper that takes the virtual address returned from dma_alloc_attrs/coherent and creates a dma_addr_t for the given new device. If the original allocation was a coherent one no cache flushing is required either (because the arch made sure it is coherent), if the original allocation used DMA_ATTR_NON_CONSISTENT the new allocation will need dma_cache_sync calls as well. - you never even try to share a mapping retourned from dma_map_resource - instead each device using it creates a new mapping, which makes sense as no virtual addresses are involved at all. > So maybe a list of (struct page *, dma_addr_t, num_pages) would suit best, > with struct page * being optional (if it's a resource, or something else > that the kernel core mm isn't aware of). But that only has benefits if we > really roll it out everywhere, in all the subsystems and drivers, since if > we don't we've made the struct pages ** <-> sgt conversion fun only worse > by adding a 3 representation of gpu buffer object backing storage. I think the most important thing about such a buffer object is that it can distinguish the underlying mapping types. While dma_alloc_coherent, dma_alloc_attrs with DMA_ATTR_NON_CONSISTENT, dma_map_page/dma_map_single/dma_map_sg and dma_map_resource all give back a dma_addr_t they are in now way interchangable. And trying to stuff them all into a structure like struct scatterlist that has no indication what kind of mapping you are dealing with is just asking for trouble.
