Re: [PATCH v2 7/8] gpu: nova-core: add NVKV typed decoding

[John Hubbard <jhubbard@nvidia.com>]

On 9/13/26 11:55 PM, Eliot Courtney wrote:
> On Mon Sep 14, 2026 at 12:46 PM JST, Alexandre Courbot wrote:
>> On Thu Aug 27, 2026 at 11:12 PM JST, Eliot Courtney wrote:
...
>>> +            fn visit(
>>> +                &mut self,
>>> +                key: $crate::gsp::nvkv::KeyId,
>>> +                index: $crate::gsp::nvkv::Index,
>>> +                value: $crate::gsp::nvkv::DecoderValue<'_>,
>>> +            ) -> ::kernel::error::Result<bool> {
>>> +                Ok(false
>>> +                    $( || $crate::gsp::nvkv::Schema::visit(&mut self.$field, key, index, value)? )*)
>>
>> Mmm looks like this is going to be `O(n)` with `n` being the number of
>> fields?
>>
>> This is ok for a first implementation but eventually I hope we can
>> switch to a more efficient dispatch.
> 
> I thought quite a bit about this while writing this code, since we need
> the escape hatch to imperative decode (custom Schema impl basically). To
> be able to get it down to a match on the key, we need to know ahead of
> time which keys a Schema will consume. That duplicates the info from the
> visit() implementation.
> 
> I thought up a few methods but it's unclear to me which one is best, so
> I just left it for now. Please LMK if you think this is urgent, I can
> try in a follow up to improve this. Here are my ideas (when I say O(1)
> lookup I mean modulo how the compiler decides to do it with the set of
> key IDs it gets):
> 
> 1. current code - just visit()
> pros: key source of truth not duplicates
> cons: O(field) visit as you say
> 

Something about the visit pattern has always concerned me, and I think
it has to do with the fact that people have been told to use it in
all kinds of situations. But it's really only ideal for a few situations,
such as updated a bunch of objects on a display, for example.

It's not a good fit here IMHO.

Is it hard to do one of the other choices, up front? If it's easy,
it would be nice to skip this visit entirely. :)

thanks,
-- 
John Hubbard