Nouveau - Sep 2025 - [PATCH v2 01/10] gpu: nova-core: Set correct DMA mask

On 9/22/25 1:30 PM, Alistair Popple wrote:
> +        // SAFETY: No DMA allocations have been made yet
It's not really about DMA allocations that have been made previously, there
is
no unsafe behavior in that.

It's about the method must not be called concurrently with any DMA
allocation or
mapping primitives.

Can you please adjust the comment correspondingly?
> +        unsafe {
pdev.dma_set_mask_and_coherent(DmaMask::new::<47>())? };
As Boqun mentioned, we shouldn't have a magic number for this. I don't
know if
it will change for future chips, but maybe we should move this to gpu::Spec to
be safe.

At least, create a constant for it (also in gpu::Spec?); in Nouveau I named this
NOUVEAU_VA_SPACE_BITS back then. Not a great name, if you have a better idea,
please go for it. :)

On 9/22/25 9:08 AM, Danilo Krummrich wrote:
> On 9/22/25 1:30 PM, Alistair Popple wrote:
>> +        // SAFETY: No DMA allocations have been made yet
> 
> It's not really about DMA allocations that have been made previously,
there is
> no unsafe behavior in that.
> 
> It's about the method must not be called concurrently with any DMA
allocation or
> mapping primitives.
> 
> Can you please adjust the comment correspondingly?
> 
>> +        unsafe {
pdev.dma_set_mask_and_coherent(DmaMask::new::<47>())? };
> 
> As Boqun mentioned, we shouldn't have a magic number for this. I
don't know if
> it will change for future chips, but maybe we should move this to gpu::Spec
to
It changes to 52 bits for GH100+ (Hopper/Blackwell+). When I post those
patches, I'll use a HAL to select the value.
> be safe.
> 
> At least, create a constant for it (also in gpu::Spec?); in Nouveau I named
this
> NOUVEAU_VA_SPACE_BITS back then. Not a great name, if you have a better
idea,
> please go for it. :)
GPU_DMA_BIT_WIDTH, for now?


thanks,
-- 
John Hubbard