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

[Arunpravin Paneer Selvam <arunpravin.paneerselvam@amd.com>]

Hi Christian,

On 10/13/2025 6:48 PM, Christian König wrote:
> 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?
I pushed this patch into drm-misc-next.

Regards,
Arun.
>
> 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);