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

[Daniel Vetter <daniel@ffwll.ch>]

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.
> 
> Fixes: c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Rob Herring <robh@kernel.org>
> ---
> v2:
> - Move subtracting the fake offset out of mmap() obj callbacks.
> 
> I've tested shmem, but not ttm. Hopefully, I understood what's needed 
> for TTM.
> 
> Rob
> 
>  drivers/gpu/drm/drm_gem.c              | 3 +++
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ---
>  drivers/gpu/drm/ttm/ttm_bo_vm.c        | 7 +++++++
>  include/drm/drm_gem.h                  | 4 +++-
>  4 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 56f42e0f2584..2f2b889096b0 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1106,6 +1106,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
>  		return -EINVAL;
>  
>  	if (obj->funcs && obj->funcs->mmap) {
> +		/* Remove the fake offset */
> +		vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> +
>  		ret = obj->funcs->mmap(obj, vma);
>  		if (ret)
>  			return ret;
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index a878c787b867..e8061c64c480 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -542,9 +542,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>  	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>  	vma->vm_ops = &drm_gem_shmem_vm_ops;
>  
> -	/* Remove the fake offset */
> -	vma->vm_pgoff -= drm_vma_node_start(&shmem->base.vma_node);
> -
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_mmap);
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 1a9db691f954..08902c7290a5 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -482,6 +482,13 @@ EXPORT_SYMBOL(ttm_bo_mmap);
>  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;
>  }
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index e71f75a2ab57..c56cbb3509e0 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -159,7 +159,9 @@ struct drm_gem_object_funcs {
>  	 *
>  	 * The callback is used by by both drm_gem_mmap_obj() and
>  	 * drm_gem_prime_mmap().  When @mmap is present @vm_ops is not
> -	 * used, the @mmap callback must set vma->vm_ops instead.
> +	 * used, the @mmap callback must set vma->vm_ops instead. The @mmap
> +	 * callback is always called with a 0 offset. The caller will remove
> +	 * the fake offset as necessary.
>  	 *

Maybe remove this empty comment line here while at it. With that

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I think I'll follow up with a patch to annotate drm_gem_mmap_obj as
deprecated and that instead this here should be used.
-Daniel

>  	 */
>  	int (*mmap)(struct drm_gem_object *obj, struct vm_area_struct *vma);
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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