Nouveau - Nov 2025 - [PATCH v7 4/4] gpu: nova-core: add boot42 support for next-gen GPUs

Hi John,

On 11/11/2025 11:30 PM, John Hubbard wrote:
> NVIDIA GPUs are moving away from using NV_PMC_BOOT_0 to contain
> architecture and revision details, and will instead use NV_PMC_BOOT_42
> in the future. NV_PMC_BOOT_0 will contain a specific set of values
> that will mean "go read NV_PMC_BOOT_42 instead".
> 
> Change the selection logic in Nova so that it will claim Turing and
> later GPUs. This will work for the foreseeable future, without any
> further code changes here, because all NVIDIA GPUs are considered, from
> the oldest supported on Linux (NV04), through the future GPUs.
[...]
> diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
> index cd58040b681b..8c5f46f6aaac 100644
> --- a/drivers/gpu/nova-core/gpu.rs
> +++ b/drivers/gpu/nova-core/gpu.rs
> @@ -175,19 +175,41 @@ pub(crate) struct Spec {
>  
>  impl Spec {
>      fn new(bar: &Bar0) -> Result<Spec> {
> +        // Some brief notes about boot0 and boot42, in chronological
order:
> +        //
> +        // NV04 through NV50:
> +        //
> +        //    Not supported by Nova. boot0 is necessary and sufficient to
identify these GPUs.
> +        //    boot42 may not even exist on some of these GPUs.
> +        //
> +        // Fermi through Volta:
> +        //
> +        //     Not supported by Nova. boot0 is still sufficient to
identify these GPUs, but boot42
> +        //     is also guaranteed to be both present and accurate.
> +        //
> +        // Turing and later:
> +        //
> +        //     Supported by Nova. Identified by first checking boot0 to
ensure that the GPU is not
> +        //     from an earlier (pre-Fermi) era, and then using boot42 to
precisely identify the GPU.
> +        //     Somewhere in the Rubin timeframe, boot0 will no longer have
space to add new GPU IDs.
> +
>          let boot0 = regs::NV_PMC_BOOT_0::read(bar);
>  
> -        Spec::try_from(boot0)
> +        if boot0.is_older_than_fermi() {
> +            return Err(ENOTSUPP);
> +        }
> +
> +        Spec::try_from(regs::NV_PMC_BOOT_42::read(bar))
There is an inconsistency in error return here, if NV04 through NV50, it returns
-ENOTSUPP. For Fermi through Volta, it will read boot42 but will return -ENODEV
because `Spec::try_from()` -> `boot42.chipset()` with return -ENODEV. I am Ok
with either error return, but it would be good to make it consistent.

There also does not seem to be a diagnostic if the chipset is not supported. It
would be good diagnostic that the chipset did not match, right now it will
return -ENODEV, which could mean the device does not exist. -ENOTSUPP is better
though but an actual dmesg error message would be nice.

With these,

Reviewed-by: Joel Fernandes <joelagnelf at nvidia.com>

Thanks.

On 11/13/25 11:59 AM, Joel Fernandes wrote:
> Hi John,
> 
...
>> -        Spec::try_from(boot0)
>> +        if boot0.is_older_than_fermi() {
>> +            return Err(ENOTSUPP);
>> +        }
>> +
>> +        Spec::try_from(regs::NV_PMC_BOOT_42::read(bar))
> 
> There is an inconsistency in error return here, if NV04 through NV50, it
returns
> -ENOTSUPP. For Fermi through Volta, it will read boot42 but will return
-ENODEV
> because `Spec::try_from()` -> `boot42.chipset()` with return -ENODEV. I
am Ok
> with either error return, but it would be good to make it consistent.
> 
Yes, good catch. It should be -ENOTSUPP for sure.
> There also does not seem to be a diagnostic if the chipset is not
supported. It
> would be good diagnostic that the chipset did not match, right now it will
> return -ENODEV, which could mean the device does not exist. -ENOTSUPP is
better
> though but an actual dmesg error message would be nice.
Yes. The "not supported" case would happen in two situations:

a) Someone found a pre-Fermi GPU to use for (probably) display, and they
also have a Turing+ GPU in the same system (!), and they have both Nouveau
and Nova drivers available.

Here, it's not really an error situation. If this actually works, then
Nova not supporting the older GPU is just expected operation.

But these older GPUs are not even really directly supported, so this
is a fringe case anyway.

b) A newer GPU is installed, and Nova does not yet support it. Here,
an error message is OK, because Nova is eventually (soon) going to 
support that GPU.

So I think that means an error message is reasonable here.
> 
> With these,
> 
> Reviewed-by: Joel Fernandes <joelagnelf at nvidia.com>
> 
Thanks for the review!

Alex, I think I'd better re-spin and re-test, in order to safely collect
the various small fixes from you and Joel. I can do that today.

thanks,
-- 
John Hubbard