linux-kernel - Re: [PATCH v3 15/16] rust: platform: add basic platform device / driver abstractions

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Message-ID: <CAL_JsqLohzzxDrJPFiQ6v8X=2i7pPUJdwzVLxShbcX-SCz_3Jg@mail.gmail.com>
Date: Tue, 26 Nov 2024 13:15:59 -0600
From: Rob Herring <robh@...nel.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: 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, aliceryhl@...gle.com, 
	airlied@...il.com, fujita.tomonori@...il.com, lina@...hilina.net, 
	pstanner@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, 
	daniel.almeida@...labora.com, saravanak@...gle.com, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v3 15/16] rust: platform: add basic platform device /
 driver abstractions

On Tue, Nov 26, 2024 at 9:17 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On Tue, Nov 26, 2024 at 08:44:19AM -0600, Rob Herring wrote:
> > > > > > The DT type and name fields are pretty much legacy, so I don't think the
> > > > > > rust bindings need to worry about them until someone converts Sparc and
> > > > > > PowerMac drivers to rust (i.e. never).
> > > > > >
> > > > > > I would guess the PCI cases might be questionable, too. Like DT, drivers
> > > > > > may be accessing the table fields, but that's not best practice. All the
> > > > > > match fields are stored in pci_dev, so why get them from the match
> > > > > > table?
> > > > >
> > > > > Fair question, I'd like to forward it to Greg. IIRC, he explicitly requested to
> > > > > make the corresponding struct pci_device_id available in probe() at Kangrejos.
> >
> > Making it available is not necessarily the same thing as passing it in
> > via probe.
>
> IIRC, that was exactly the request.
>
> > I agree it may need to be available in probe(), but that
> > can be an explicit call to get it.
>
> Sure, I did exactly that for the platform abstraction, because there we may
> probe through different ID tables.

TBC, I think of_match_device() (both calling the C API and the method)
should not be part of this series. I think we agreed on that already.
Only if there is a need at some point later should we add it.

> A `struct pci_driver`'s probe function has the following signature [1] though:
>
> `int (*probe)(struct pci_dev *dev, const struct pci_device_id *id)`
>
> [1] https://elixir.bootlin.com/linux/v6.12/source/include/linux/pci.h#L950

We have a mixture of probe with and without the _device_id parameter.
I'd question if we really want to keep that for PCI when we have a
chance to align things with Rust. We can't really with C as it would
be too many drivers to change. Passing the _device_id only works if
firmware matching is never used which can change over time. But if
aligning things is not something we want to do, then I'll shut up.

> > > > Which table gets passed in though? Is the IdInfo parameter generic and
> > > > can be platform_device_id, of_device_id or acpi_device_id? Not sure if
> > > > that's possible in rust or not.
> > >
> > > Not sure I can follow you here.
> > >
> > > The `IdInfo` parameter is of a type given by the driver for driver specific data
> > > for a certain ID table entry.
> > >
> > > It's analogue to resolving `pci_device_id::driver_data` in C.
> >
> > As I said below, the PCI case is simpler than for platform devices.
> > Platform devices have 3 possible match tables. The *_device_id type we
> > end up with is determined at runtime (because matching is done at
> > runtime), so IdInfo could be any of those 3 types.
>
> `IdInfo` is *not* any of the three *_device_id types. It's the type of the
> drivers private data associated with an entry of any of the three ID tables.

Ah yes, indeed. So no issue with the probe method.

> It is true that a driver, which registers multiple out of those three tables is
> currently forced to have the same private data type for all of them.

I think that's a feature actually as it enforces best practices.

> I don't think this is a concern, is it? If so, it's easily resolvable by just
> adding two more associated types, e.g. `PlatformIdInfo`, `DtIdInfo` and
> `AcpiIdInfo`.
>
> In this case we would indeed need accessor functions like `dt_match_data`,
> `platform_match_data`, `acpi_match_data`, since we don't know the type at
> compile time anymore.

Do we need to split those out in rust or can we just call
device_get_match_data()?

>
> I don't think that's necessary though.

Even if you don't support all 3 tables now, at a minimum I think you
need to rename things to be clear what table type is supported and
allow for adding the other types. For example, T::ID_TABLE needs to be
renamed to be clear it's the of_device_id table.

Rob