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

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

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?

> 
> 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/
>