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

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Message-Id: <DDUUUUL0GXAM.2LNWCIEW124JY@nvidia.com>
Date: Wed, 29 Oct 2025 22:54:19 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>, "John Hubbard"
 <jhubbard@...dia.com>
Cc: "Alexandre Courbot" <acourbot@...dia.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, "Edwin Peer" <epeer@...dia.com>, "Zhi Wang"
 <zhiw@...dia.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, "Bjorn Helgaas" <bhelgaas@...gle.com>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>, "LKML"
 <linux-kernel@...r.kernel.org>, "Nouveau"
 <nouveau-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH v3 2/2] gpu: nova-core: add boot42 support for next-gen
 GPUs

On Wed Oct 29, 2025 at 8:26 PM JST, Danilo Krummrich wrote:
<snip>
>> @@ -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
> 	    }
> 	)

I don't think this will work because `NV_PMC_BOOT_0` and
`NV_PMC_BOOT_42` are different types, so we cannot alternate them in the
same call to `try_from`. But the following should:

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

    if boot0.is_nv04() {
        Err(ENODEV)?
    }

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