Re: [PATCH 01/10] gpu: nova-core: Set correct DMA mask

[John Hubbard <jhubbard@nvidia.com>]

On 9/1/25 4:55 PM, Alistair Popple wrote:
> On 2025-08-30 at 09:55 +1000, John Hubbard <jhubbard@nvidia.com> wrote...
>> On 8/27/25 1:19 AM, Alistair Popple wrote:
...
>>>   
>>> +        // SAFETY: No DMA allocations have been made yet
>>> +        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<48>())? };
>>
>> Eventually, should be 52 bits wide, rather than 48. Or so I believe from
>> looking at various drivers, including Nouveau (which uses 52-bit for
>> Blackwell) and mlx* (which use a 64-bit mask).
>>
>> However, it works for the current series, because this series only supports
>> Ampere GPUs, and 48 bits suffices for those.
> 
> Actually based on both Nouveau and our internal docs this should be 47-bits. I

Yes. Which is why I wrote "48 bits suffices".

> suspect I just chose 48 during initial bring-up because that's what most CPUs
> support but neglected to add the TODO to actually go and check this. So will fix
> for v2.
> 
>> So, you could leave this patch as-is, and I'll change 48 --> 52 in the
>> upcoming Hopper/Blackwell series. Or you can change it now.
> 
> We can't of course just change this to 52 bits because this needs to reflect
> what the GPU HW supports. So ideally this needs to come from the HAL. I left

I should have been more precise. I meant, "use 52 bits, via HAL, just
like Nouveau does".

> this hard-coded because in the short-term leaving it as 47 bits even for
> Blackwell won't cause any issues. It may force usage of an IOMMU to address
> physical addresses greater than 47-bits when it otherwise wouldn't for
> Hopper/Blackwell (it would always have to for Ampere/Turing), but short-term I
> doubt many systems actually have physical memory above 47-bits anyway.
> 
> In other words you could leave this as 47 bits in the upcoming Hopper/Blackwell
> series or use the HAL we have come up with (if that is available) to obtain the
> optimal value.

Yes. I'm planning to match Nouveau's HAL approach for this, in the
upcoming Hopper/Blackwell series.



thanks,
-- 
John Hubbard