linux-kernel - Re: [PATCH 03/10] device property: Add fwnode_property_read_int

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Message-ID: <Z-7LcXoGw7uNWBUE@smile.fi.intel.com>
Date: Thu, 3 Apr 2025 20:54:57 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Rob Herring <robh@...nel.org>
Cc: Remo Senekowitsch <remo@...nzli.dev>,
	Daniel Scally <djrscally@...il.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	devicetree@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 03/10] device property: Add
 fwnode_property_read_int_array()

On Thu, Apr 03, 2025 at 11:36:38AM -0500, Rob Herring wrote:
> On Thu, Apr 3, 2025 at 11:15 AM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > On Thu, Apr 03, 2025 at 11:04:32AM -0500, Rob Herring wrote:
> > > On Thu, Mar 27, 2025 at 3:41 AM Andy Shevchenko
> > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > On Wed, Mar 26, 2025 at 06:13:42PM +0100, Remo Senekowitsch wrote:
> > > > > The rust bindings for reading device properties has a single
> > > > > implementation supporting differing sizes of integers. The fwnode C API
> > > > > already has a similar interface, but it is not exposed with the
> > > > > fwnode_property_ API. Add the fwnode_property_read_int_array() wrapper.

...

> > > > > +EXPORT_SYMBOL_GPL(fwnode_property_read_int_array);
> > > >
> > > > I'm not sure about this. We have a lot of assumptions in the code that the
> > > > arrays beneath are only represented by the selected number of integer types.
> > > > This opens a Pandora's box, e.g., reading in u24, which is not supported by
> > > > the upper layers..
> > >
> > > We can probably drop the export if it is just that which you object to.
> >
> > Yes, this is main point, but dropping it does not prevent from still using in
> > the complied-in code. Is it possible to hide it better?
> 
> Don't put any declaration in the header and declare it in the rust
> code? But lack of declaration generates warnings.

Exactly. And I believe we have the typed versions of int_array for a reason.
Otherwise what's the point in having them to begin with?
(The first what comes to my mind is a compile time type checking, so we don't
 try to load u8 with u32 data or any other dirty tricks.)

Maybe it deserves a header that can be included explicitly in the rust stuff
and being checked at compile time to avoid people using that? Can we achieve
something like C preprocessor

#ifndef FOO
#error This header must not be included directly
#endif

> Also, all the backends will reject an arbitrary size. So your worry
> about u24 or other odd sizes isn't really valid. But if you want to be
> doubly paranoid for when we add a new firmware backend (shoot me now),
> you could move this from the swnode implementation to the fwnode
> implementation:
> 
>         if (!is_power_of_2(elem_size) || elem_size > sizeof(u64))
>                 return -ENXIO;

That might work. But still an interface of int_array seems lower by
level than typed ones.

-- 
With Best Regards,
Andy Shevchenko