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

[Alexandre Courbot]

On Wed Mar 25, 2026 at 10:38 PM JST, Danilo Krummrich wrote:
> 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.

Yeah, I guess that's sufficient indeed. John, my apologies for the
back-and-forth.