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

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

On 2025-09-23 at 12:16 +1000, John Hubbard <jhubbard at nvidia.com>
wrote...
> 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?
Sure.
> >> +        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. :)
Well it's certainly not the VA_SPACE width ... that's a different
address space :-)

I thought from the context that the magic number was pretty obviously the
supported DMA address width in bits, especially given the extra decoration
of the DmaMask type. Certainly that's been the accepted practice for the
rest
of the kernel where pretty much all drivers just use something of the form
dma_set_mask(drm_dev->dev, DMA_BIT_MASK(44)) or whatever DMA address widths
they support.
> GPU_DMA_BIT_WIDTH, for now?
Works for me.
> thanks,
> -- 
> John Hubbard
>