[PATCH v2] drm/ttm: Add safety check for NULL man->bdev in ttm_resource_manager_usage

[Jesse.Zhang <Jesse.Zhang@amd.com>]

The `ttm_resource_manager_usage()` function currently assumes `man->bdev` is non-NULL when accessing `man->bdev->lru_lock`.
However, in scenarios where the resource manager is not fully initialized (e.g., APU platforms that lack dedicated VRAM, or incomplete manager setup),
 `man->bdev` may remain NULL. This leads to a NULL pointer dereference when attempting to acquire the `lru_lock`, triggering kernel OOPS.

Fix this by adding an explicit safety check for `man->bdev` before accessing its members:
- Use `WARN_ON_ONCE(!man->bdev)` to emit a one-time warning (a soft assertion) when `man->bdev` is NULL. This helps catch invalid usage patterns during debugging without breaking production workflows.
- Return 0 immediately if `man->bdev` is NULL, as a non-initialized manager cannot have valid resource usage to report.

Suggested-by: Christian König <christian.koenig@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
---
 drivers/gpu/drm/ttm/ttm_resource.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
index e2c82ad07eb4..d93d1bef6768 100644
--- a/drivers/gpu/drm/ttm/ttm_resource.c
+++ b/drivers/gpu/drm/ttm/ttm_resource.c
@@ -587,6 +587,9 @@ uint64_t ttm_resource_manager_usage(struct ttm_resource_manager *man)
 {
 	uint64_t usage;
 
+	if (WARN_ON_ONCE(!man->bdev))
+		return 0;
+
 	spin_lock(&man->bdev->lru_lock);
 	usage = man->usage;
 	spin_unlock(&man->bdev->lru_lock);
-- 
2.49.0