Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

Christian König
2020-09-17T12:25:03Z

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.

For TTM drivers vma->vm_private_data points to the buffer object. Not
sure about the drivers using GEM only.

Why are you asking?

Regards,
Christian.

>
> Jason
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