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.
Why are you asking?
I'm thinking about using find_vma on something that is not
get_user_pages()'able to go to the underlying object, in this case dma
buf.
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.
Some applications are braindead enough to mmap() a buffer and then give
us back the CPU pointer and request to make it a handle (userptr) again.
That is clearly forbidden by OpenGL, OpenCL and Vulkan, but they use it
anyway.
Christian.
Jason
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