linux-kernel - [PATCH v2] drm/cma-helper: Set VM

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Date:   Wed, 13 Oct 2021 10:36:54 -0400
From:   Alyssa Rosenzweig <alyssa@...enzweig.io>
To:     dri-devel@...ts.freedesktop.org
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>
Subject: [PATCH v2] drm/cma-helper: Set VM_DONTEXPAND for mmap

From: Robin Murphy <robin.murphy@....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@....com>
Reviewed-and-tested-by: Alyssa Rosenzweig <alyssa@...enzweig.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