Re: [PATCH] rust: DmaFence: Add better warning through Device reference

[Philipp Stanner <phasta@mailbox.org>]

On Fri, 2026-09-25 at 14:30 +0200, Danilo Krummrich wrote:
> On Fri Sep 25, 2026 at 12:26 PM CEST, Gary Guo wrote:
> > On Fri Sep 25, 2026 at 9:19 AM BST, Philipp Stanner wrote:
> > > Replace the warning print with a dev_warn!(). To do so, have the
> > > FenceContext carry a reference to a Device, protected by the already
> > > present lifetime.
> > 
> > Add a lifetime just to do this a print doesn't sound ideal. You could use
> > `ARef<Device>` instead?
> 
> Yes, that's what I recommend in general.

Weren't you super opposed to refcounting wherever it's avoidable?


[…]

> That said, we can also use WARN_ON() instead, which avoids the device dependency
> to begin with and still provides enough information to find the "offender".

WARN_ON() is fine by me.

> 
> But as I mentioned previously, I don't consider this that bad of a condition in
> the first place. Signaling with ECANCELED on drop() seems perfectly reasonable:
> 
> When a driver does a teardown of the channel (or more generically the execution
> context) it will follow the RAII pattern, so it will be very natural to just
> drop the Jobqueue, which will drop all jobs and hence all DriverFence objects.
> 
> IOW, driver will likely invent a new type that does the same thing on drop, just
> without the warning. :)

The driver can avoid dropping half-forgotten stuff by calling
jobqueue.complete_all_jobs(ECANCELED) immediately before dropping,
which allows us for having the warning without false-positives.


P.