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

["Christian König" <christian.koenig@amd.com>]

On 13.10.25 10:58, Jesse.Zhang wrote:
> 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>

Reviewed-by: Christian König <christian.koenig@amd.com>

@Arun could you push this one to drm-misc-next if nobody objects?

Thanks in advance,
Christian

> ---
>  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);