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.
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?
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
Jason