drm/ttm: Work around performance regression with VM_PFNMAP

A performance regression was introduced in TTM in linux 3.13 when we started using
VM_PFNMAP for shared mappings. In theory this should've been faster due to
less page book-keeping but it appears like VM_PFNMAP + x86 PAT + write-combine
is a particularly cpu-hungry combination, as seen by largely increased
cpu-usage on r200 GL video playback.

Until we've sorted out why, revert to always use VM_MIXEDMAP.
Reference: freedesktop.org bugzilla bug #75719

Reported-and-tested-by: <smoki00790@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: stable@vger.kernel.org

1 files changed, 7 insertions, 5 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 801231c9ae48..0ce48e5a9cb4 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -339,11 +339,13 @@ int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma,
         vma->vm_private_data = bo;
 
         /*
-         * PFNMAP is faster than MIXEDMAP due to reduced page
-         * administration. So use MIXEDMAP only if private VMA, where
-         * we need to support COW.
+         * We'd like to use VM_PFNMAP on shared mappings, where
+         * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
+         * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
+         * bad for performance. Until that has been sorted out, use
+         * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
          */
-        vma->vm_flags |= (vma->vm_flags & VM_SHARED) ? VM_PFNMAP : VM_MIXEDMAP;
+        vma->vm_flags |= VM_MIXEDMAP;
         vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
         return 0;
 out_unref:
@@ -359,7 +361,7 @@ int ttm_fbdev_mmap(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
 
         vma->vm_ops = &ttm_bo_vm_ops;
         vma->vm_private_data = ttm_bo_reference(bo);
-        vma->vm_flags |= (vma->vm_flags & VM_SHARED) ? VM_PFNMAP : VM_MIXEDMAP;
+        vma->vm_flags |= VM_MIXEDMAP;
         vma->vm_flags |= VM_IO | VM_DONTEXPAND;
         return 0;
 }