[PATCH] drm/ttm: Add NULL check in ttm_resource_manager_usage

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

Add a NULL pointer check in ttm_resource_manager_usage() to prevent
kernel NULL pointer dereferences when the function is called with
an uninitialized resource manager.

This fixes a kernel OOPS observed on APU devices where the VRAM
resource manager is not fully initialized, but various sysfs and
debug interfaces still attempt to query VRAM usage statistics.

The crash backtrace showed:
    BUG: kernel NULL pointer dereference, address: 00000000000008f8
    Call Trace:
     amdttm_resource_manager_usage+0x1f/0x40 [amdttm]
     amdgpu_mem_info_vram_used_show+0x1e/0x40 [amdgpu]
     dev_attr_show+0x1d/0x40
     kernfs_seq_show+0x27/0x30

By returning 0 for NULL managers, we allow callers to safely query
usage information even when the underlying resource manager is not
available, which is the expected behavior for devices without
dedicated VRAM like APUs.

Suggested-by: Philip Yang <Philip.Yang@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..e4d45f75e40a 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 (!man)
+		return 0;
+
 	spin_lock(&man->bdev->lru_lock);
 	usage = man->usage;
 	spin_unlock(&man->bdev->lru_lock);
-- 
2.49.0