Re: [PATCH v2] gpu: Fix uninitialized buddy for built-in drivers

[Matthew Auld <matthew.auld@intel.com>]

On 19/02/2026 12:56, Danilo Krummrich wrote:
> On Thu Feb 19, 2026 at 1:44 PM CET, Matthew Auld wrote:
>> On 19/02/2026 11:14, Danilo Krummrich wrote:
>>> On Thu Feb 19, 2026 at 11:38 AM CET, Matthew Auld wrote:
>>>> On 19/02/2026 10:16, Danilo Krummrich wrote:
>>>>> On Mon Feb 16, 2026 at 12:19 PM CET, Koen Koning wrote:
>>>>>> Use subsys_initcall instead of module_init for the GPU buddy allocator,
>>>>>> so its initialization code runs before any gpu drivers.
>>>>>> Otherwise, a built-in driver that tries to use the buddy allocator will
>>>>>> run into a kernel NULL pointer dereference because slab_blocks is
>>>>>> uninitialized.
>>>>>>
>>>>>> Specifically, this fixes drm/xe (as built-in) running into a kernel
>>>>>> panic during boot, because it uses buddy during device probe.
>>>>>
>>>>> I just noticed that this patch was sent twice, and I posted my feedback on [1]
>>>>> -- pasting it here as well.
>>>>>
>>>>>> Fixes: ba110db8e1bc ("gpu: Move DRM buddy allocator one level up (part two)")
>>>>>
>>>>> This Fixes: tag seems wrong. How is this code move related to this problem?
>>>>
>>>> This popped up as a very recent regression for us internally. It looks
>>>> like it worked before since link order ensured drm_buddy came before all
>>>> the driver code. With above commit the link order changed and became
>>>> drm/ and then buddy. See [1] also, which is maybe clearer to see this.
>>>
>>> I see, I think this would be a good hint for the commit message. :)
>>>
>>> However, I think it was never meant to rely on a build system implementation
>>> detail, nor would this be correct. So, I think this should add both Fixes: tags.
>>
>> Yeah, I'm really not sure tbh. From a quick grep there do seem to be
>> other users relying on this:
>>
>> drm/drm_drv.c:1274:module_init(drm_core_init);
>> drm/scheduler/sched_fence.c:238:module_init(drm_sched_fence_slab_init);
>>
>> The sched one looks identical with the slab thing. Do these need to be
>> fixed also?
> 
> Yes, those should be fixed as well.
> 
> Also note that module_init() compiles down to device_initcall() when built-in,
> i.e. the initcall stage that is mainly for drivers, not for subsystem code.
> 
> Do you want to send a fix for thise as well?

Koen will send something.

> 
>>>
>>> Whether it should be backported is a different question though, as it seems to
>>> work by accident in previous versions, i.e. it is only a "potential bug".
>>>
>>> My personal opinion is that it should be backported either way, however that's
>>> ultimately up to the stable team.
>>>
>>> - Danilo
>>>
>>>>
>>>> [1]
>>>> https://lore.kernel.org/intel-xe/20260213152047.179628-1-koen.koning@linux.intel.com/
>>>>
>>>>>
>>>>> This should rather be:
>>>>>
>>>>> Fixes: 6387a3c4b0c4 ("drm: move the buddy allocator from i915 into common drm")
>>>>>
>>>>> Also, please add:
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>>
>>>>>> Cc: Joel Fernandes <joelagnelf@nvidia.com>
>>>>>> Cc: Dave Airlie <airlied@redhat.com>
>>>>>> Cc: intel-xe@lists.freedesktop.org
>>>>>> Cc: Peter Senna Tschudin <peter.senna@linux.intel.com>
>>>>>> Cc: Matthew Auld <matthew.auld@intel.com>
>>>>>> Signed-off-by: Koen Koning <koen.koning@linux.intel.com>
>>>>>
>>>>> [1] https://lore.kernel.org/all/DGIUUFLC31D5.2OZBF5FWQJWMZ@kernel.org/
>>>
>