Re: [PATCH v3 01/13] gpu: nova-core: Set correct DMA mask

[John Hubbard]

On 9/30/25 6:42 PM, Alistair Popple wrote:
> On 2025-09-30 at 23:29 +1000, Danilo Krummrich <dakr@xxxxxxxxxx> wrote...
>> On 9/30/25 3:16 PM, Alistair Popple wrote:
>>> +        // SAFETY: No DMA allocations have been made yet
>>> +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<47>())? };
>>
>> I think you forgot to derive the value from the relevant sources, i.e. physical
>> bus, DMA controller and MMU capabilities.
>>
>> I assume not all GPU architectures / generations have the exact same capabilities?
> 
> Right. Long term we need a HAL for this, and I believe John was going to look
> at that. In the short term everything we care about supports the same 47-bit

Yes. This will arrive with the Hopper/Blackwell support patchset.

> address width. And when I say "everything" I mean Turing and Ampere, although
> I'm pretty sure 47-bits goes back to at least Pascal.
> 
> Newer GPU architectures (Hopper+) support greater widths (52-bits), but there's
> no real impact to constraining these for now until a proper HAL is in place.

Yes. The "soul" of these various patchsets so far is, "get GSP responsive
for Ampere/Ada". To that end, it's OK to temporarily act as if the only
thing that matters in this driver is Ampere/Ada. :)

> 
> And that sounds like an excellent addition that I should make to the commit
> logs and/or the comment on the constant definition, which fell off my radar when
> reworking the rest of this series but will fix for v4.

thanks,
-- 
John Hubbard