[PATCH] drm/dma-helpers: Don't change vma flags

[Daniel Vetter]

This code was added in b65e64f7ccd4 ("drm/cma: Use
dma_mmap_writecombine() to mmap buffer"), but does not explain why
it's needed.

It should be entirely unnecessary, because remap_pfn_range(), which is
what the various dma_mmap functiosn are built on top of, does already
unconditionally adjust the vma flags:

https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518

More importantly, it does uncondtionally set VM_PFNMAP, so clearing
that does not make much sense.

Patch motived by discussions around enforcing VM_PFNMAP semantics for
all dma-buf users, where Thomas asked why dma helpers will work with
that dma_buf_mmap() contract.

References: https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6fbb7@suse.de/
Cc: Laurent Pinchart <laurent.pinchart+renesas at ideasonboard.com>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
Cc: "Christian König" <christian.koenig at amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c b/drivers/gpu/drm/drm_gem_dma_helper.c
index 1e658c448366..637a5cc62457 100644
--- a/drivers/gpu/drm/drm_gem_dma_helper.c
+++ b/drivers/gpu/drm/drm_gem_dma_helper.c
@@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object *dma_obj, struct vm_area_struct *
 	int ret;
 
 	/*
-	 * 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.
+	 * Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
+	 * want to map the whole buffer.
 	 */
 	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
-	vma->vm_flags &= ~VM_PFNMAP;
-	vma->vm_flags |= VM_DONTEXPAND;
 
 	if (dma_obj->map_noncoherent) {
 		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
-- 
2.37.2