[PATCH 1/3] drm/xe/oa: Move default oa unit assignment earlier during stream open

[Ashutosh Dixit <ashutosh.dixit@intel.com>]

De-referencing param.oa_unit, when an OA unit id is not provided during
stream open, results in NPD below.

  Oops: general protection fault, probably for non-canonical address...
  KASAN: null-ptr-deref in range...
  RIP: 0010:xe_oa_stream_open_ioctl+0x169/0x38a0
   xe_observation_ioctl+0x19f/0x270
   drm_ioctl_kernel+0x1f4/0x410

Fix this by moving default oa unit assignment before the dereference.

Reported-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6840
Fixes: c7e269aa565f ("drm/xe/oa: Allow exec_queue's to be specified only for OAG OA unit")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
---
 drivers/gpu/drm/xe/xe_oa.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_oa.c b/drivers/gpu/drm/xe/xe_oa.c
index cc48663c2b48f..46a79b60585b8 100644
--- a/drivers/gpu/drm/xe/xe_oa.c
+++ b/drivers/gpu/drm/xe/xe_oa.c
@@ -1962,10 +1962,6 @@ static int xe_oa_assign_hwe(struct xe_oa *oa, struct xe_oa_open_param *param)
 	enum xe_hw_engine_id id;
 	int ret = 0;
 
-	/* If not provided, OA unit defaults to OA unit 0 as per uapi */
-	if (!param->oa_unit)
-		param->oa_unit = &xe_root_mmio_gt(oa->xe)->oa.oa_unit[0];
-
 	/* When we have an exec_q, get hwe from the exec_q */
 	if (param->exec_q) {
 		param->hwe = xe_gt_hw_engine(param->exec_q->gt, param->exec_q->class,
@@ -2031,6 +2027,10 @@ int xe_oa_stream_open_ioctl(struct drm_device *dev, u64 data, struct drm_file *f
 	if (ret)
 		return ret;
 
+	/* If not provided, OA unit defaults to OA unit 0 as per uapi */
+	if (!param.oa_unit)
+		param.oa_unit = &xe_root_mmio_gt(oa->xe)->oa.oa_unit[0];
+
 	if (param.exec_queue_id > 0) {
 		/* An exec_queue is only needed for OAR/OAC functionality on OAG */
 		if (XE_IOCTL_DBG(oa->xe, param.oa_unit->type != DRM_XE_OA_UNIT_TYPE_OAG))
-- 
2.48.1