On Thu, Apr 19, 2018 at 01:16:57AM -0700, Christoph Hellwig wrote: > On Mon, Apr 16, 2018 at 03:38:56PM +0200, Daniel Vetter wrote: > > We've broken that assumption in i915 years ago. Not struct page backed > > gpu memory is very real. > > > > Of course we'll never feed such a strange sg table to a driver which > > doesn't understand it, but allowing sg_page == NULL works perfectly > > fine. At least for gpu drivers. > > For GPU drivers on x86 with no dma coherency problems, sure. But not > all the world is x86. We already have problems due to dmabugs use > of the awkward get_sgtable interface (see the common on > arm_dma_get_sgtable that I fully agree with), and doing this for memory > that doesn't have a struct page at all will make things even worse. x86 dma isn't coherent either, if you're a GPU :-) Flushing gpu caches tends to be too expensive, so there's pci-e support and chipset support to forgo it. Plus drivers flushing caches themselves. The dma_get_sgtable thing is indeed fun, right solution would probably be to push the dma-buf export down into the dma layer. The comment for arm_dma_get_sgtable is also not a realy concern, because dma-buf also abstracts away the flushing (or well is supposed to), so there really shouldn't be anyone calling the streaming apis on the returned sg table. That's why dma-buf gives you an sg table that's mapped already. > > If that's not acceptable then I guess we could go over the entire tree > > and frob all the gpu related code to switch over to a new struct > > sg_table_might_not_be_struct_page_backed, including all the other > > functions we added over the past few years to iterate over sg tables. > > But seems slightly silly, given that sg tables seem to do exactly what > > we need. > > It isn't silly. We will have to do some surgery like that anyway > because the current APIs don't work. So relax, sit back and come up > with an API that solves the existing issues and serves us well in > the future. So we should just implement a copy of sg table for dma-buf, since I still think it does exactly what we need for gpus? Yes there's a bit a layering violation insofar that drivers really shouldn't each have their own copy of "how do I convert a piece of dma memory into dma-buf", but that doesn't render the interface a bad idea. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
