Re: [PATCH v3 2/2] gpu: nova-core: add boot42 support for next-gen GPUs

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Oct 29, 2025 at 4:03 AM CET, John Hubbard wrote:
> diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
> index 6f1486d4e9c6..6762493206ec 100644
> --- a/drivers/gpu/nova-core/gpu.rs
> +++ b/drivers/gpu/nova-core/gpu.rs
> @@ -134,6 +134,34 @@ pub(crate) struct Revision {
>      minor: u8,
>  }
>  
> +impl TryFrom<regs::NV_PMC_BOOT_0> for Spec {
> +    type Error = Error;
> +
> +    fn try_from(boot0: regs::NV_PMC_BOOT_0) -> Result<Self> {
> +        Ok(Self {
> +            chipset: boot0.chipset()?,
> +            revision: Revision {
> +                major: boot0.major_revision(),
> +                minor: boot0.minor_revision(),
> +            },

Actually, would be nice to handle this just like chipset for consistency, i.e.

	revision: boot0.revision()

> +        })
> +    }
> +}
> +
> +impl TryFrom<regs::NV_PMC_BOOT_42> for Spec {
> +    type Error = Error;
> +
> +    fn try_from(boot42: regs::NV_PMC_BOOT_42) -> Result<Self> {
> +        Ok(Self {
> +            chipset: boot42.chipset()?,
> +            revision: Revision {
> +                major: boot42.major_revision(),
> +                minor: boot42.minor_revision(),
> +            },

Same here, could be

	revision: boot42.revision()

> +        })
> +    }
> +}
> +
>  impl fmt::Display for Revision {
>      fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
>          write!(f, "{:x}.{:x}", self.major, self.minor)
> @@ -151,13 +179,43 @@ impl Spec {
>      fn new(bar: &Bar0) -> Result<Spec> {
>          let boot0 = regs::NV_PMC_BOOT_0::read(bar);
>  
> -        Ok(Self {
> -            chipset: boot0.chipset()?,
> -            revision: Revision {
> -                major: boot0.major_revision(),
> -                minor: boot0.minor_revision(),
> -            },
> -        })
> +        // "next-gen" GPUs (some time after Blackwell) will zero out boot0, and put the architecture
> +        // details in boot42 instead. Avoid reading boot42 unless we are in that case.
> +        let boot42 = if boot0.is_next_gen() {
> +            Some(regs::NV_PMC_BOOT_42::read(bar))
> +        } else {
> +            None
> +        };
> +
> +        // Some brief notes about boot0 and boot42, in chronological order:
> +        //
> +        // NV04 through Volta:
> +        //
> +        //    Not supported by Nova. boot0 is necessary and sufficient to identify these GPUs.
> +        //    boot42 may not even exist on some of these GPUs.boot42
> +        //
> +        // Turing through Blackwell:
> +        //
> +        //     Supported by both Nouveau and Nova. boot0 is still necessary and sufficient to
> +        //     identify these GPUs. boot42 exists on these GPUs but we don't need to use it.
> +        //
> +        // Future "next-gen" GPUs:
> +        //
> +        //    Only supported by Nova. boot42 has the architecture details, boot0 is zeroed out.
> +
> +        // NV04, the very first NVIDIA GPU to be supported on Linux, is identified by a specific bit
> +        // pattern in boot0. Although Nova does not support NV04 (see above), it is possible to
> +        // confuse NV04 with a "next-gen" GPU. Therefore, return early if we specifically detect
> +        // NV04, thus simplifying the remaining selection logic.
> +        if boot0.is_nv04() {
> +            Err(ENODEV)?
> +        }
> +
> +        // Now that we know it is something more recent than NV04, use boot42 if we previously
> +        // determined that boot42 was both valid and relevant, and boot0 otherwise.
> +        boot42
> +            .map(Spec::try_from)
> +            .unwrap_or_else(|| Spec::try_from(boot0))
>      }
>  }

Without the comments this currently is:

	let boot42 = if boot0.is_next_gen() {
	    Some(regs::NV_PMC_BOOT_42::read(bar))
	} else {
	    None
	};
	
	if boot0.is_nv04() {
	    Err(ENODEV)?
	}
	
	boot42
	    .map(Spec::try_from)
	    .unwrap_or_else(|| Spec::try_from(boot0))

Which I think is a bit heavy-handed. Let's simplify this a bit:

	let boot0 = regs::NV_PMC_BOOT_0::read(bar);

	if boot0.is_nv04() {
	    return Err(ENODEV);
	}

	Spec::try_from(
	    if boot0.is_next_gen() {
	        regs::NV_PMC_BOOT_42::read(bar)
	    } else {
	        boot0
	    }
	)