From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
The FAULT_FLAG_ALLOW_RETRY semantics is tricky and appears poorly
documented. Add a comment to the TTM fault() implementation to avoid
future confusion.
Cc: Christian Koenig <christian.koenig@xxxxxxxxxx>
Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
---
drivers/gpu/drm/ttm/ttm_bo_vm.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index a61a35e57d1c..e93b1ad7828f 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -125,6 +125,17 @@ static vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
&bdev->man[bo->mem.mem_type];
struct vm_area_struct cvma;
+ /*
+ * FAULT_FLAG_ALLOW_RETRY indicates that the caller would like to avoid
+ * IO waits with the mmap_sem held. With FAULT_FLAG_RETRY_NOWAIT,
+ * the caller would like an early return with VM_FAULT_RETRY and the
+ * mmap_sem held.
+ * If FAULT_FLAG_RETRY_NOWAIT is not set, then we should drop the
+ * mmap_sem, perform the wait and return VM_FAULT_RETRY without
+ * holding it. We implement this for sleeping bo reservations
+ * where the reservation holder may be waiting for fences, and for
+ * fence waits.
+ */
if (unlikely(!dma_resv_trylock(bo->base.resv))) {
if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
--
2.20.1
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