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

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

On Sat Oct 25, 2025 at 2:14 AM CEST, John Hubbard wrote:
> +        // "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
> +        };
> +
>          try_pin_init!(Self {
>              chipset: {
> -                let chipset = boot0.chipset()?;
> +                // 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.
> +                //
> +                // 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.
> +                let (chipset, major_rev, minor_rev) = if let Some(boot42) = boot42 {
> +                    (
> +                        boot42.chipset()?,
> +                        boot42.major_revision(),
> +                        boot42.minor_revision(),
> +                    )
> +                } else {
> +                    // Current/older GPU: use BOOT0
> +                    (
> +                        boot0.chipset()?,
> +                        boot0.major_revision(),
> +                        boot0.minor_revision(),
> +                    )
> +                };

Why open code all of the above in the struct Gpu constructor? This could all
happen within Spec::new().

If we *really* don't want to store the Spec, but only the Chipset, you can also
do:

	try_pin_init!(Self {
	    chipset: {
	        let spec = Spec::new(bar);

	        dev_info!(pdev.as_ref(), "{}\n", spec);

	        spec.chipset
	    },
	    [...],
	}