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

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Message-ID: <41b48a39-a408-4968-a98d-17e771f290e8@nvidia.com>
Date: Wed, 29 Oct 2025 18:13:45 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Timur Tabi <ttabi@...dia.com>, "dakr@...nel.org" <dakr@...nel.org>
Cc: "lossin@...nel.org" <lossin@...nel.org>,
 "a.hindborg@...nel.org" <a.hindborg@...nel.org>,
 "boqun.feng@...il.com" <boqun.feng@...il.com>, Zhi Wang <zhiw@...dia.com>,
 "simona@...ll.ch" <simona@...ll.ch>, "tmgross@...ch.edu"
 <tmgross@...ch.edu>, "alex.gaynor@...il.com" <alex.gaynor@...il.com>,
 "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
 "ojeda@...nel.org" <ojeda@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
 "bjorn3_gh@...tonmail.com" <bjorn3_gh@...tonmail.com>,
 Edwin Peer <epeer@...dia.com>, "airlied@...il.com" <airlied@...il.com>,
 "aliceryhl@...gle.com" <aliceryhl@...gle.com>,
 "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
 Joel Fernandes <joelagnelf@...dia.com>,
 Alexandre Courbot <acourbot@...dia.com>, "gary@...yguo.net"
 <gary@...yguo.net>, Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH v3 2/2] gpu: nova-core: add boot42 support for next-gen
 GPUs

On 10/29/25 6:07 PM, John Hubbard wrote:
> On 10/29/25 6:01 PM, Timur Tabi wrote:
>> On Wed, 2025-10-29 at 17:35 -0700, John Hubbard wrote:
>>>          // "Future" GPUs (some time after Rubin) will set 
>>> `architecture_0`
>>>          // to 0, and `architecture_1` to 1, and put the architecture 
>>> details in
>>>          // boot42 instead.
>>
>> I don't want to kick a dead horse here, but aren't the architecture 
>> details already in boot42
>> for Turing?  I thought the whole point was that we don't need boot0 
>> any more, and only Nouveau
>> has to worry about boot0 vs boot42.
> 
> Yes, but someone can still plug in a pre-Turing GPU and try to
> boot up with nova-core.ko on the system.
> 
> So it's important to avoid getting into trouble in that case.

In fact, here's what I have staged for the next posting:

impl Spec { 

     fn new(bar: &Bar0) -> Result<Spec> { 

         // 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.
         // 

         // Rubin: 

         // 

         //     Only supported by Nova. Need to use boot42 to fully 
identify these GPUs.
         // 

         // "Future" (after Rubin) GPUs: 

         // 

         //    Only supported by Nova. NV_PMC_BOOT's ARCH_0 (bits 28:24) 
will be zeroed out, and
         //    ARCH_1 (bit 8:8) will be set to 1, which will mean, 
"refer to NV_PMC_BOOT_42".
  

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

  

         if boot0.use_boot42_instead() { 

             Spec::try_from(regs::NV_PMC_BOOT_42::read(bar)) 

         } else { 

             Spec::try_from(boot0) 

         } 

     } 

}