Re: [PATCH 07/31] gpu: nova-core: set DMA mask width based on GPU architecture

[John Hubbard <jhubbard@nvidia.com>]

On 1/13/26 5:43 AM, Gary Guo wrote:
> On Wed Dec 3, 2025 at 5:58 AM GMT, John Hubbard wrote:
...
>> +impl Architecture {
>> +    /// Returns the number of DMA address bits supported by this architecture.
>> +    ///
>> +    /// Hopper and Blackwell support 52-bit DMA addresses, while earlier architectures
>> +    /// (Turing, Ampere, Ada) support 47-bit DMA addresses.
>> +    pub(crate) const fn dma_addr_bits(&self) -> u32 {
> 
> How about just return `DmaMask` from here? This get rids of the fallible
> constructor call of `DmaMask`.

Yes, that's a nice touch, I'll do that.

> 
>> +        match self {
>> +            Self::Turing | Self::Ampere | Self::Ada => 47,
>> +            Self::Hopper | Self::Blackwell => 52,
>> +        }
>> +    }
>> +}
>> +
>>  impl TryFrom<u8> for Architecture {
>>      type Error = Error;
>>  
>> @@ -203,6 +216,20 @@ pub(crate) struct Spec {
>>      revision: Revision,
>>  }
>>  
>> +/// Reads the GPU architecture from BAR0 registers.
>> +///
>> +/// This is a lightweight check used early in probe to determine the correct DMA address width
>> +/// before the full [`Spec`] is constructed.
>> +pub(crate) fn read_architecture(bar: &Bar0) -> Result<Architecture> {
>> +    let boot0 = regs::NV_PMC_BOOT_0::read(bar);
>> +
>> +    if boot0.is_older_than_fermi() {
>> +        return Err(ENODEV);
>> +    }
>> +
>> +    regs::NV_PMC_BOOT_42::read(bar).architecture()
> 
> Can this just be `Spec::new`?

Yes it can. I've fixed it locally, to do Spec::new() early and pass
the Spec into Gpu::new(), and it does result in less code here. Good.


thanks,
-- 
John Hubbard