linux-kernel - Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

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Date:   Thu, 9 Mar 2023 18:11:18 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     Asahi Lina <lina@...hilina.net>, 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>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, Arnd Bergmann <arnd@...db.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Neal Gompa <neal@...pa.dev>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

On Thu, Mar 09, 2023 at 01:46:39PM -0300, Wedson Almeida Filho wrote:
> On Thu, 9 Mar 2023 at 08:24, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> > > > > +        // owns a reference. This is satisfied by the call to `get_device` above.
> > > > > +        Self { ptr }
> > > > > +    }
> > > > > +
> > > > > +    /// Creates a new device instance from an existing [`RawDevice`] instance.
> > > > > +    pub fn from_dev(dev: &dyn RawDevice) -> Self {
> > > >
> > > > I am a rust newbie, but I don't understand this "RawDevice" here at all.
> > >
> > > Different buses will have their own Rust "Device" type, for example,
> > > pci::Device, amba::Device, platform::Device that wrap their C
> > > counterparts pci_dev, amba_device, platform_device.
> > >
> > > "RawDevice" is a trait for functionality that is common to all
> > > devices. It exposes the "struct device" of each bus/subsystem so that
> > > functions that work on any "struct device", for example, `clk_get`,
> > > `pr_info`. will automatically work on all subsystems.
> >
> > Why is this being called "Raw" then?  Why not just "Device" to follow
> > along with the naming scheme that the kernel already uses?
> 
> Because it gives us access to underlying raw `struct device` pointer,
> in Rust raw pointers are those unsafe `*mut T` or `*const T`. I'm not
> married to the name though, we should probably look for a better one
> if this one is confusing.
> 
> Just "Device" is already taken. It's a ref-counted `struct device` (it
> calls get_device/put_device in the right places automatically,
> guarantees no dandling pointers); it is meant to be used by code that
> needs to hold on to devices when they don't care about the bus. (It in
> fact implements `RawDevice`.)

I don't understand, why do you need both of these?  Why can't one just
do?  Why would you need one without the other?  I would think that
"Device" and "RawDevice" here would be the same thing, that is a way to
refer to a "larger" underlying struct device memory chunk in a way that
can be passed around without knowing, or caring, what the "real" device
type is.

>  How about `IsDevice`?

That sounds like a question, and would return a boolean, not a structure :)

> Then, for example, the platform bus would implement `IsDevice` for
> `plaform::Device`.

I don't really understand that, sorry.

thanks,

greg k-h