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    Subject[PATCH v2] drm/cma-helper: Set VM_DONTEXPAND for mmap
    Date
    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.

    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
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