[PATCH v2 4/5] drm/xe: Guard against NULL GT in xe_guc.c

[Jonathan Cavitt <jonathan.cavitt@intel.com>]

Static analysis reveals the following issue:
xe_device_get_gt is theoretically able to return NULL in some cases, but
several use cases don't check the return value before performing a
dereference, resulting in a NULL pointer dereference.

Use xe_assert to assert root_gt is not NULL before dereferencing it.

While we're here, replace xe_device_get_gt(xe, 0) with the equivalent
call xe_root_mmio_gt(xe).  There's no reason to use the flexible
xe_device_get_gt function when we're only always tring to get GT0 here.

v2: xe_assert root_gt is not NULL (Michal, Matt)

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
---
 drivers/gpu/drm/xe/xe_guc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_guc.c b/drivers/gpu/drm/xe/xe_guc.c
index d5adbbb013ec..38fba439f1e8 100644
--- a/drivers/gpu/drm/xe/xe_guc.c
+++ b/drivers/gpu/drm/xe/xe_guc.c
@@ -466,10 +466,13 @@ static int guc_g2g_alloc(struct xe_guc *guc)
 		return 0;
 
 	if (gt->info.id != 0) {
-		struct xe_gt *root_gt = xe_device_get_gt(xe, 0);
-		struct xe_guc *root_guc = &root_gt->uc.guc;
+		struct xe_gt *root_gt = xe_root_mmio_gt(xe);
+		struct xe_guc *root_guc;
 		struct xe_bo *bo;
 
+		xe_assert(xe, root_gt);
+		root_guc = &root_gt->uc.guc;
+
 		bo = xe_bo_get(root_guc->g2g.bo);
 		if (!bo)
 			return -ENODEV;
-- 
2.43.0