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Subject[PATCH 2/2] drm/vgem: use normal cached mmap'ings
From: Rob Clark <>

Since there is no real device associated with vgem, it is impossible to
end up with appropriate dev->dma_ops, meaning that we have no way to
invalidate the shmem pages allocated by vgem. So, at least on platforms
without drm_cflush_pages(), we end up with corruption when cache lines
from previous usage of vgem bo pages get evicted to memory.

The only sane option is to use cached mappings.

Signed-off-by: Rob Clark <>
Possibly we could dma_sync_*_for_{device,cpu}() on dmabuf attach/detach,
although the ->gem_prime_{pin,unpin}() API isn't quite ideal for that as
it is. And that doesn't really help for drivers that don't attach/
detach for each use.

But AFAICT vgem is mainly used for dmabuf testing, so maybe we don't
need to care too much about use of cached mmap'ings.

drivers/gpu/drm/vgem/vgem_drv.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
index 11a8f99ba18c..ccf0c3fbd586 100644
--- a/drivers/gpu/drm/vgem/vgem_drv.c
+++ b/drivers/gpu/drm/vgem/vgem_drv.c
@@ -259,9 +259,6 @@ static int vgem_mmap(struct file *filp, struct vm_area_struct *vma)
if (ret)
return ret;

- /* Keep the WC mmaping set by drm_gem_mmap() but our pages
- * are ordinary and not special.
- */
vma->vm_flags = flags | VM_DONTEXPAND | VM_DONTDUMP;
return 0;
@@ -382,7 +379,7 @@ static void *vgem_prime_vmap(struct drm_gem_object *obj)
if (IS_ERR(pages))
return NULL;

- return vmap(pages, n_pages, 0, pgprot_writecombine(PAGE_KERNEL));
+ return vmap(pages, n_pages, 0, PAGE_KERNEL);

static void vgem_prime_vunmap(struct drm_gem_object *obj, void *vaddr)
@@ -411,7 +408,7 @@ static int vgem_prime_mmap(struct drm_gem_object *obj,
vma->vm_file = get_file(obj->filp);
vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
- vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
+ vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);

return 0;
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