Re: [PATCH v2] drm/cma-helper: Set VM_DONTEXPAND for mmap

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Oct 13, 2021 at 10:36:54AM -0400, Alyssa Rosenzweig wrote:
> From: Robin Murphy <robin.murphy@arm.com>
> 
> drm_gem_cma_mmap() cannot assume every implementation of dma_mmap_wc()
> will end up calling remap_pfn_range() (which happens to set the relevant
> vma flag, among others), so in order to make sure expectations around
> VM_DONTEXPAND are met, let it explicitly set the flag like most other
> GEM mmap implementations do.
> 
> This avoids repeated warnings on a small minority of systems where the
> display is behind an IOMMU, and has a simple driver which does not
> override drm_gem_cma_default_funcs. Arm hdlcd is an in-tree affected
> driver. Out-of-tree, the Apple DCP driver is affected; this fix is
> required for DCP to be mainlined.

How/where does this warn? Also there should be a lot more drivers than
just these two which have an iommu for the display block, so this not
working is definitely a more wide-spread issue.
-Daniel

> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> Reviewed-and-tested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
> ---
>  drivers/gpu/drm/drm_gem_cma_helper.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index d53388199f34..63e48d98263d 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -510,6 +510,7 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>  	 */
>  	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
>  	vma->vm_flags &= ~VM_PFNMAP;
> +	vma->vm_flags |= VM_DONTEXPAND;
>  
>  	cma_obj = to_drm_gem_cma_obj(obj);
>  
> -- 
> 2.30.2
> 

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