Re: [PATCH v8 05/31] gpu: nova-core: set DMA mask width based on GPU architecture

[Danilo Krummrich]

On Wed Mar 25, 2026 at 12:45 PM CET, Alexandre Courbot wrote:
> On Wed Mar 25, 2026 at 8:31 PM JST, Danilo Krummrich wrote:
>> On Wed Mar 25, 2026 at 4:52 AM CET, John Hubbard wrote:
>>> Move Spec creation into probe() so the architecture is known before
>>> setting the DMA mask, and pass Spec into Gpu::new().
>>
>> Hm...this is not what we discussed in [1]. You implemented the change in v5 and
>> v6 and v7 reverted it. Why?
>>
>> [1] https://lore.kernel.org/all/DGC28K8J39E2.1X173NHT9ZJDI@xxxxxxxxxx/
>
> That was on my feedback [1].
>
> `Gpu::new` carries no guarantee in itself that no DMA ops have been
> performed so far; whereas in `probe` this is much clearer.

That's not what has to be justified. Having DMA allocations before setting the
mask is not a violation of the safety contract; it is about concurrent
allocations.

That said, all DMA allocations are on the Gpu object, so there can't be
concurrent DMA allocations while we are still in the constructor of the Gpu
object.

Unless...one would try to create a second Gpu object from probe() (which
obviously makes zero sense). nova-core already relies on Gpu to be unique per
device, as attempting to create a second Gpu object causes UB and actual memory
corruption when there is no IOMMU. :)

[ 123.966391] pci 0000:01:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x000e address=0xf7fef000 flags=0x0000]

So, I think technically Gpu::new() already has the safety requirement that it
must not be constructed more than once per device.

I wonder if we want probe() to give out a ProbeToken<'a>, so a driver type can
consume it by value to guarantee uniqueness of an object without unsafe.