Re: [PATCH v9 0/5] rust: DebugFS Bindings

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Jul 09, 2025 at 03:04:51PM -0700, Matthew Maurer wrote:
> On Wed, Jul 9, 2025 at 2:59 PM Danilo Krummrich <dakr@kernel.org> wrote:
> >
> > On Wed Jul 9, 2025 at 11:53 PM CEST, Matthew Maurer wrote:
> > > On Wed, Jul 9, 2025 at 2:47 PM Danilo Krummrich <dakr@kernel.org> wrote:
> > >>
> > >> On Wed Jul 9, 2025 at 9:09 PM CEST, Matthew Maurer wrote:
> > >> > This series provides safe DebugFS bindings for Rust, with a sample
> > >> > module using them.
> > >> >
> > >> > Example interaction with the sample driver:
> > >>
> > >> I understand what you're trying to do here, i.e. showcase that values exported
> > >> via debugfs can be altered.
> > >>
> > >> The problem is that the current abstractions only implement read(), but not
> > >> write().
> > >
> > > I was trying to keep the initial bindings simple. Adding `write` is
> > > definitely something we could do, but I thought maybe that could be in
> > > a subsequent patch.
> >
> > Absolutely, yes! I didn't mean to ask to add it now. :)
> >
> > >> If you really want to showcase changing values, you can, for instance, create a
> > >> workqueue inside the sample driver and modify the counter periodically.
> > >
> > > This is supposed to be sample code, so ideally it should be as narrow
> > > as is reasonable in what subsystems it touches, no? If people would
> > > really prefer the sample schedule a ticking counter I can do that, but
> > > it already felt weird to be registering a platform driver in a debugfs
> > > sample.
> >
> > I'm not asking to do that. If the values don't change for now, because
> > there's no write() yet, that's perfectly fine with me. :)
> 
> Potentially I misinterpreted Greg[1], I thought he wanted to see how
> mutation would work.
> If we don't need mutation, I'm fine simplifying the driver to not have
> any mutation triggers and just export a few random things.

I mean, the most simple way would be to create the debugfs entries in probe()
and mutate them - still in probe() - right afterwards once. I think we should
do in any case. And AFAICT, this also covers [1].

> [1]: https://lore.kernel.org/all/2025070349-tricky-arguable-5362@gregkh/
> 
> >
> > >>
> > >> We really should not teach people to modify values by read() instead of write().
> > >> Also, without this workaround there shouldn't be a reason to export the exact
> > >> same value twice, i.e. no need for File<File<AtomicUsize>>.
> > >>
> > >> - Danilo
> > >
> > > How do you feel about the `Wrapper` struct, intended to simulate the
> > > driver doing its actual job and show how that would look? Is that
> > > similarly verboten, even though there's a comment on it saying this
> > > isn't how one should do things?
> >
> > Yeah, let's not do that -- don't give people ideas. :)