On Thu, Sep 17, 2020 at 5:24 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Thu, Sep 17, 2020 at 04:54:44PM +0200, Christian König wrote: > > Am 17.09.20 um 16:35 schrieb Jason Gunthorpe: > > > On Thu, Sep 17, 2020 at 02:24:29PM +0200, Christian König wrote: > > > > Am 17.09.20 um 14:18 schrieb Jason Gunthorpe: > > > > > On Thu, Sep 17, 2020 at 02:03:48PM +0200, Christian König wrote: > > > > > > Am 17.09.20 um 13:31 schrieb Jason Gunthorpe: > > > > > > > On Thu, Sep 17, 2020 at 10:09:12AM +0200, Daniel Vetter wrote: > > > > > > > > > > > > > > > Yeah, but it doesn't work when forwarding from the drm chardev to the > > > > > > > > dma-buf on the importer side, since you'd need a ton of different > > > > > > > > address spaces. And you still rely on the core code picking up your > > > > > > > > pgoff mangling, which feels about as risky to me as the vma file > > > > > > > > pointer wrangling - if it's not consistently applied the reverse map > > > > > > > > is toast and unmap_mapping_range doesn't work correctly for our needs. > > > > > > > I would think the pgoff has to be translated at the same time the > > > > > > > vm->vm_file is changed? > > > > > > > > > > > > > > The owner of the dma_buf should have one virtual address space and FD, > > > > > > > all its dma bufs should be linked to it, and all pgoffs translated to > > > > > > > that space. > > > > > > Yeah, that is exactly like amdgpu is doing it. > > > > > > > > > > > > Going to document that somehow when I'm done with TTM cleanups. > > > > > BTW, while people are looking at this, is there a way to go from a VMA > > > > > to a dma_buf that owns it? > > > > Only a driver specific one. > > > Sounds OK > > > > > > > For TTM drivers vma->vm_private_data points to the buffer object. Not sure > > > > about the drivers using GEM only. > > > Why are drivers in control of the vma? I would think dma_buf should be > > > the vma owner. IIRC module lifetime correctness essentially hings on > > > the module owner of the struct file > > > > Because the page fault handling is completely driver specific. > > > > We could install some DMA-buf vmops, but that would just be another layer of > > redirection. Uh geez I didn't know amdgpu was doing that :-/ Since this is on, I guess the inverse of trying to convert a userptr into a dma-buf is properly rejected? > If it is already taking a page fault I'm not sure the extra function > call indirection is going to be a big deal. Having a uniform VMA > sounds saner than every driver custom rolling something. > > When I unwound a similar mess in RDMA all the custom VMA stuff in the > drivers turned out to be generally buggy, at least. > > Is vma->vm_file->private_data universally a dma_buf pointer at least? Nope. I think if you want this without some large scale rewrite of a lot of code we'd need a vmops->get_dmabuf or similar. Not pretty, but would get the job done. > > > So, user VA -> find_vma -> dma_buf object -> dma_buf operations on the > > > memory it represents > > > > Ah, yes we are already doing this in amdgpu as well. But only for DMA-bufs > > or more generally buffers which are mmaped by this driver instance. > > So there is no general dma_buf service? That is a real bummer Mostly historical reasons and "it's complicated". One problem is that dma-buf isn't a powerful enough interface that drivers could use it for all their native objects, e.g. userptr doesn't pass through it, and clever cache flushing tricks aren't allowed and a bunch of other things. So there's some serious roadblocks before we could have a common allocator (or set of allocators) behind dma-buf. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
