Re: [PATCH] gpu: nova-core: fb: make sure to unregister SysmemFlush on boot failure

["Eliot Courtney" <ecourtney@nvidia.com>]

On Mon Apr 13, 2026 at 7:55 PM JST, Danilo Krummrich wrote:
> On Fri Apr 10, 2026 at 5:57 PM CEST, Gary Guo wrote:
>>> +impl Drop for SysmemFlush {
>>> +    fn drop(&mut self) {
>>> +        let _ = self.bar.try_access_with(|bar| self.unregister(bar));
>>
>> I feel that this is the wrong solution to the problem.
>
> Yeah, it is pretty fragile as it relies on outer implementation details, such as
> the fact that SysmemFlush is part of the device private data and hence
> try_access_with() will fail when not dropped in an unwind path.
>
>> The thing we want is to *ensure* that `SysmemFlush` Drop is called with device
>> still being bound.
>>
>> It's not yet fully clear to me how we'd want to guarantee that, but one API that
>> might make sense is to create a DevRes API that allows you to reference an
>> existing `DevRes` and have driver-core making sure that the tear down happens in
>> reverse order. So inside the `Drop` the `bar` can still be unconditionally
>> access.
>
> Yes, I have something like this on my list of things I want to look into for
> while (my list entry calls it DevresChain).
>
> I want to leverage the internal reference count of Devres for this, which is not
> exactly straight forward, but should be possible.
>
> I will prioritize this and have a look.

Yeah, I agree that this patch is the wrong long term solution. If we
don't have the infra for the proper way to do it soon, it might be worth
taking it anyway since it adds minimal complexity and fixes a real
issue.

I had a brief look into the Devres chain stuff that exists on the C side
and it looks like it doesn't provide any actual guarantees about the
lifetime (it seems possible to delete from the middle of the chain, so
descendants can't assume ancestors exist in general, AFAICT), so the
the translation of that into lifetimes in rust might get interesting

thanks

>
> Thanks,
> Danilo