Re: [PATCH 3/5] drm: Add and export function drm_gem_cma_mmap_noncoherent

[Christoph Hellwig <hch@infradead.org>]

On Mon, Nov 02, 2020 at 10:06:49PM +0000, Paul Cercueil wrote:
> This function can be used by drivers that need to mmap dumb buffers
> created with non-coherent backing memory.
> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> ---
>  drivers/gpu/drm/drm_gem_cma_helper.c | 39 ++++++++++++++++++++++++++++
>  include/drm/drm_gem_cma_helper.h     |  2 ++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 3bdd67795e20..4ed63f4896bd 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -387,6 +387,45 @@ int drm_gem_cma_mmap(struct file *filp, struct vm_area_struct *vma)
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_cma_mmap);
>  
> +/**
> + * drm_gem_cma_mmap_noncoherent - memory-map a CMA GEM object with
> + *     non-coherent cache attribute
> + * @filp: file object
> + * @vma: VMA for the area to be mapped
> + *
> + * Just like drm_gem_cma_mmap, but for a GEM object backed by non-coherent
> + * memory.
> + *
> + * Returns:
> + * 0 on success or a negative error code on failure.
> + */
> +int drm_gem_cma_mmap_noncoherent(struct file *filp, struct vm_area_struct *vma)
> +{
> +	struct drm_gem_cma_object *cma_obj;
> +	int ret;
> +
> +	ret = drm_gem_mmap(filp, vma);
> +	if (ret)
> +		return ret;
> +
> +	cma_obj = to_drm_gem_cma_obj(vma->vm_private_data);
> +
> +	/*
> +	 * Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
> +	 * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
> +	 * the whole buffer.
> +	 */
> +	vma->vm_flags &= ~VM_PFNMAP;
> +	vma->vm_pgoff = 0;
> +	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> +
> +	return remap_pfn_range(vma, vma->vm_start,
> +			       cma_obj->paddr >> PAGE_SHIFT,
> +			       vma->vm_end - vma->vm_start,
> +			       vma->vm_page_prot);

Per patch 1 cma_obj->paddr is the dma address, while remap_pfn_range
expects a physical address.  This does not work.