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)
}
