[PATCH 2/2] drm/xe/vm: make PREFETCH to TT sticky

[Matthew Auld <matthew.auld@intel.com>]

When userspace explicitly requests DRM_XE_VM_BIND_OP_PREFETCH to system
memory (XE_PL_TT), xe_bo_migrate() moves the buffer object to TT.
However, because XE_PL_TT is marked with TTM_PL_FLAG_FALLBACK in the
buffer's default placement (introduced in commit 5c87fee3c96c ("drm/xe:
Attempt to bring bos back to VRAM after eviction")), any subsequent
validation during exec immediately migrates the buffer back to VRAM,
completely undoing the userspace prefetch request.

Use xe_bo_migrate_tt_sticky() when prefetching to XE_PL_TT so that the
buffer remains resident in system memory across subsequent validations.

Assisted-by: LLM
Reported-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 5c87fee3c96c ("drm/xe: Attempt to bring bos back to VRAM after eviction")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.12+
---
 drivers/gpu/drm/xe/xe_vm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index 425c678f4480..2068e0cf50ff 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -3406,7 +3406,10 @@ static int op_lock_and_prep(struct drm_exec *exec, struct xe_vm *vm,
 				mem_type = region_to_mem_type[region];
 			}
 
-			err = xe_bo_migrate(bo, mem_type, NULL, exec);
+			if (mem_type == XE_PL_TT)
+				err = xe_bo_migrate_tt_sticky(bo, NULL, exec);
+			else
+				err = xe_bo_migrate(bo, mem_type, NULL, exec);
 		}
 		break;
 	}
-- 
2.55.0