Message-ID: <Z1mpvRDnlZN9Mc5o@cassiopeiae>
Date: Wed, 11 Dec 2024 16:03:25 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg KH <gregkh@...uxfoundation.org>, rafael@...nel.org,
	bhelgaas@...gle.com, ojeda@...nel.org, alex.gaynor@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, tmgross@...ch.edu, a.hindborg@...sung.com,
	airlied@...il.com, fujita.tomonori@...il.com, lina@...hilina.net,
	pstanner@...hat.com, ajanulgu@...hat.com, lyude@...hat.com,
	robh@...nel.org, daniel.almeida@...labora.com, saravanak@...gle.com,
	dirk.behme@...bosch.com, j@...nau.net, fabien.parent@...aro.org,
	chrisi.schrefl@...il.com, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v5 01/16] rust: pass module name to `Module::init`

On Wed, Dec 11, 2024 at 03:55:47PM +0100, Alice Ryhl wrote:
> On Wed, Dec 11, 2024 at 3:52 PM Danilo Krummrich <dakr@...nel.org> wrote:
> >
> > On Wed, Dec 11, 2024 at 03:45:53PM +0100, Alice Ryhl wrote:
> > > On Wed, Dec 11, 2024 at 3:29 PM Danilo Krummrich <dakr@...nel.org> wrote:
> > > >
> > > > On Wed, Dec 11, 2024 at 02:34:54PM +0100, Alice Ryhl wrote:
> > > > > On Wed, Dec 11, 2024 at 2:31 PM Danilo Krummrich <dakr@...nel.org> wrote:
> > > > > >
> > > > > > On Wed, Dec 11, 2024 at 02:14:37PM +0100, Greg KH wrote:
> > > > > > > On Wed, Dec 11, 2024 at 01:34:31PM +0100, Danilo Krummrich wrote:
> > > > > > > > On Wed, Dec 11, 2024 at 01:22:33PM +0100, Danilo Krummrich wrote:
> > > > > > > > > On Wed, Dec 11, 2024 at 12:05:10PM +0100, Greg KH wrote:
> > > > > > > > > > On Wed, Dec 11, 2024 at 11:59:54AM +0100, Greg KH wrote:
> > > > > > > > > > > On Wed, Dec 11, 2024 at 11:48:23AM +0100, Greg KH wrote:
> > > > > > > > > > > > On Wed, Dec 11, 2024 at 11:45:20AM +0100, Greg KH wrote:
> > > > > > > > > > > > > On Tue, Dec 10, 2024 at 11:46:28PM +0100, Danilo Krummrich wrote:
> > > > > > > > > > > > > > In a subsequent patch we introduce the `Registration` abstraction used
> > > > > > > > > > > > > > to register driver structures. Some subsystems require the module name on
> > > > > > > > > > > > > > driver registration (e.g. PCI in __pci_register_driver()), hence pass
> > > > > > > > > > > > > > the module name to `Module::init`.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Nit, we don't need the NAME of the PCI driver (well, we do like it, but
> > > > > > > > > > > > > that's not the real thing), we want the pointer to the module structure
> > > > > > > > > > > > > in the register_driver call.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Does this provide for that?  I'm thinking it does, but it's not the
> > > > > > > > > > > > > "name" that is the issue here.
> > > > > > > > > > > >
> > > > > > > > > > > > Wait, no, you really do want the name, don't you.  You refer to
> > > > > > > > > > > > "module.0" to get the module structure pointer (if I'm reading the code
> > > > > > > > > > > > right), but as you have that pointer already, why can't you just use
> > > > > > > > > > > > module->name there as well as you have a pointer to a valid module
> > > > > > > > > > > > structure that has the name already embedded in it.
> > > > > > > > > > >
> > > > > > > > > > > In digging further, it's used by the pci code to call into lower layers,
> > > > > > > > > > > but why it's using a different string other than the module name string
> > > > > > > > > > > is beyond me.  Looks like this goes way back before git was around, and
> > > > > > > > > > > odds are it's my fault for something I wrote a long time ago.
> > > > > > > > > > >
> > > > > > > > > > > I'll see if I can just change the driver core to not need a name at all,
> > > > > > > > > > > and pull it from the module which would make all of this go away in the
> > > > > > > > > > > end.  Odds are something will break but who knows...
> > > > > > > > > >
> > > > > > > > > > Nope, things break, the "name" is there to handle built-in modules (as
> > > > > > > > > > the module pointer will be NULL.)
> > > > > > > > > >
> > > > > > > > > > So what you really want is not the module->name (as I don't think that
> > > > > > > > > > will be set), but you want KBUILD_MODNAME which the build system sets.
> > > > > > > > >
> > > > > > > > > That's correct, and the reason why I pass through this name argument.
> > > > > > > > >
> > > > > > > > > Sorry I wasn't able to reply earlier to save you some time.
> > > > > > > > >
> > > > > > > > > > You shouldn't need to pass the name through all of the subsystems here,
> > > > > > > > > > just rely on the build system instead.
> > > > > > > > > >
> > > > > > > > > > Or does the Rust side not have KBUILD_MODNAME?
> > > > > > > > >
> > > > > > > > > AFAIK, it doesn't (or didn't have at the time I wrote the patch).
> > > > > > > > >
> > > > > > > > > @Miguel: Can we access KBUILD_MODNAME conveniently?
> > > > > > > >
> > > > > > > > Actually, I now remember there was another reason why I pass it through in
> > > > > > > > `Module::init`.
> > > > > > > >
> > > > > > > > Even if we had env!(KBUILD_MODNAME) already, I'd want to use it from the bus
> > > > > > > > abstraction code, e.g. rust/kernel/pci.rs. But since this is generic code, it
> > > > > > > > won't get the KBUILD_MODNAME from the module that is using the bus abstraction.
> > > > > > >
> > > > > > > Rust can't do that in a macro somehow that all pci rust drivers can pull
> > > > > > > from?
> > > > > >
> > > > > > The problem is that register / unregister is encapsulated within methods of the
> > > > > > abstraction types. So the C macro trick (while generally possible) isn't
> > > > > > applicable.
> > > > > >
> > > > > > I think we could avoid having an additional `name` parameter in `Module::init`,
> > > > > > but it would still need to be the driver resolving `env!(KBUILD_MODNAME)`
> > > > > > passing it into the bus abstraction.
> > > > > >
> > > > > > However, similar to what Alice suggested in another thread, we could include
> > > > > > this step in the `module_*_driver!` macros.
> > > > > >
> > > > > > Modules that don't use this convenience macro would need to do it by hand
> > > > > > though. But that's probably not that big a deal.
> > > > >
> > > > > I think we can do it in the core `module!` macro that everyone has to use.
> > > >
> > > > How? The `module!` macro does not know about the registration instances within
> > > > the module structure.
> > >
> > > You could have the module! macro emit something along these lines:
> > >
> > > impl ModuleName for {type_} {
> > >     const NAME: &'static CStr = c_str!(env!("KBUILD_MODNAME"));
> > > }
> > >
> > > Then you can do `<Self as ModuleName>::NAME` to obtain the name elsewhere.
> >
> > Where {type_} would need to be the driver's `Driver` structure?
> >
> > We'd then need to define the bus adapter as:
> >
> > `pub struct Adapter<T: Driver + ModuleName>(T)`
> >
> > But the question stands I guess, how would the module macro know {type_}?
> 
> If you look at the macro implementation in rust/macros/module.rs you
> will find many uses of {type_} throughout the expansion. It's whatever
> is passed to the macro using the `type:` argument.

Oh, I see. So, this means that module / driver author would still need to create
the "connection" by listing the correspong driver types in the module! macro,
right?

If so, I think it'd be better to do it in the `module_*_driver!` macro and let
people implement the trait by hand for modules with multiple drivers (which
should be pretty rare).

The reason is that I think that otherwise we're probably encoding too much
semantics into the `module!` macro that isn't obvious and people need to
understand.

> 
> 
> Alice

Powered by blists - more mailing lists