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Commit 0e6d6ec0 authored by Thomas Hellstrom's avatar Thomas Hellstrom
Browse files

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: default avatar <smoki00790@gmail.com>
Acked-by: default avatarAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: default avatarThomas Hellstrom <thellstrom@vmware.com>
Cc: stable@vger.kernel.org
parent 45db98e5

drivers/gpu/drm/ttm/ttm_bo_vm.c

+7 −5
Original line number Diff line number Diff line
@@ -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;

}