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

[Robin Murphy <robin.murphy@xxxxxxx>]

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.

[Alyssa: Update commit message.]

Fixes: c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
Acked-by: Daniel Vetter <daniel@xxxxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@xxxxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20211013143654.39031-1-alyssa@xxxxxxxxxxxxx

diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
index cefd0cbf9deb..dc275c466c9c 100644
--- a/drivers/gpu/drm/drm_gem_cma_helper.c
+++ b/drivers/gpu/drm/drm_gem_cma_helper.c
@@ -512,6 +512,7 @@ int drm_gem_cma_mmap(struct drm_gem_cma_object *cma_obj, struct vm_area_struct *
 	 */
 	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
 	vma->vm_flags &= ~VM_PFNMAP;
+	vma->vm_flags |= VM_DONTEXPAND;
 
 	if (cma_obj->map_noncoherent) {
 		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);