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

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Message-ID: <Z0XmgXwwNikW6oJw@cassiopeiae>
Date: Tue, 26 Nov 2024 16:17:21 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Rob Herring <robh@...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 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.

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

> 
> > > 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.

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 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.

I don't think that's necessary though.

> Is the exact type
> opaque to probe() and will that magically work in rust? Or do we need
> to pass in the 'driver_data' ptr (or reference) itself? The matched
> driver data is generally all the driver needs or cares about. We can
> probably assume that it is the same type no matter which match table
> is used whether it is platform_device_id::driver_data,
> of_device_id::data, or acpi_device_id::driver_data. Nothing in the C
> API guarantees that, but that's just best practice. Best practice in C
> looks like this:
> 
> my_probe()
> {
>   struct my_driver_data *data = device_get_match_data();
>   ...
> }
> 
> device_get_match_data() is just a wrapper to handle the 3 possible match tables.
> 
> The decision for rust is whether we pass in "data" to probe or have an
> explicit call. There is a need to get to the *_device_id entry, but
> that's the exception. I would go as far as saying we may never need
> that in rust drivers.
> 
> Rob
> 
> > > PCI is the exception, not the rule here, in that it only matches with
> > > pci_device_id. At least I think that is the case currently, but it is
> > > entirely possible we may want to do ACPI/DT matching like every other
> > > bus. There are cases where PCI devices are described in DT.
> > >
> > > Rob
> > >
>