[PATCH v2] drm/gem: Fix mmap fake offset handling for drm_gem_object_funcs.mmap

[Gerd Hoffmann]

On Thu, Oct 24, 2019 at 02:18:59PM -0500, Rob Herring wrote:
> Commit c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
> introduced a GEM object mmap() hook which is expected to subtract the
> fake offset from vm_pgoff. However, for mmap() on dmabufs, there is not
> a fake offset.
> 
> To fix this, let's always call mmap() object callback with an offset of 0,
> and leave it up to drm_gem_mmap_obj() to remove the fake offset.
> 
> TTM still needs the fake offset, so we have to add it back until that's
> fixed.

Fixing ttm looks easy, there are not many calls to drm_vma_node_start()
the ttm code.  Can look into this when I'm back from kvm forum @ lyon.

>  int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
>  {
>  	ttm_bo_get(bo);
> +
> +	/*
> +	 * FIXME: &drm_gem_object_funcs.mmap is called with the fake offset
> +	 * removed. Add it back here until the rest of TTM works without it.
> +	 */
> +	vma->vm_pgoff += drm_vma_node_start(&bo->base.vma_node);
> +
>  	ttm_bo_mmap_vma_setup(bo, vma);
>  	return 0;
>  }

Yes, that should keep ttm happy for now.  Survived a quick smoke test
with qemu and bochs.

Acked-by: Gerd Hoffmann <kraxel at redhat.com>

cheers,
  Gerd