linux-kernel - Re: [PATCH v3 11/16] rust: pci: add basic PCI device / driver abstractions

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Message-ID: <Z0XV8R8Sbgd9mAq7@cassiopeiae>
Date: Tue, 26 Nov 2024 15:06:41 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com,
	ojeda@...nel.org, alex.gaynor@...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, robh@...nel.org,
	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 11/16] rust: pci: add basic PCI device / driver
 abstractions

On Sun, Oct 27, 2024 at 03:42:41PM -0700, Boqun Feng wrote:
> Hi Danilo,
> 
> On Tue, Oct 22, 2024 at 11:31:48PM +0200, Danilo Krummrich wrote:
> [...]
> > +/// The PCI device representation.
> > +///
> > +/// A PCI device is based on an always reference counted `device:Device` instance. Cloning a PCI
> > +/// device, hence, also increments the base device' reference count.
> > +#[derive(Clone)]
> > +pub struct Device(ARef<device::Device>);
> > +
> 
> Similar to https://lore.kernel.org/rust-for-linux/ZgG7TlybSa00cuoy@boqun-archlinux/
> 
> Could you also avoid wrapping a point to a PCI device? Instead, wrap the
> object type:

It's not wrapping a pointer, but an `ARef<device::Device>`.

> 
>     #[repr(transparent)]
>     pub struct Device(Opaque<bindings::pci_dev>);
> 
>     impl AlwaysRefCounted for Device {
>         <put_device() and get_device() on ->dev>
>     }

This implementation is currently implicit, since `pci::Device` just wraps an
`ARef<device::Device>` (like any other bus specific device structure does), and
hence increments and decrements the reference count of the underlying `struct
device` automatically.

However, what I dislike about it is that with that, `pci::Device` behaves like
an `ARef<T>`, but isn't wrapped by `ARef` itself.

Just doing what you proposed is probably cleaner is this aspect, but generates a
bit of duplicated code in reference counting the underlying `struct device`.

- Danilo

> 
> Regards,
> Boqun
> 
> > +impl Device {
> > +    /// Create a PCI Device instance from an existing `device::Device`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// `dev` must be an `ARef<device::Device>` whose underlying `bindings::device` is a member of
> > +    /// a `bindings::pci_dev`.
> > +    pub unsafe fn from_dev(dev: ARef<device::Device>) -> Self {
> > +        Self(dev)
> > +    }
> > +
> > +    fn as_raw(&self) -> *mut bindings::pci_dev {
> > +        // SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
> > +        // embedded in `struct pci_dev`.
> > +        unsafe { container_of!(self.0.as_raw(), bindings::pci_dev, dev) as _ }
> > +    }
> > +
> > +    /// Enable memory resources for this device.
> > +    pub fn enable_device_mem(&self) -> Result {
> > +        // SAFETY: `self.as_raw` is guaranteed to be a pointer to a valid `struct pci_dev`.
> > +        let ret = unsafe { bindings::pci_enable_device_mem(self.as_raw()) };
> > +        if ret != 0 {
> > +            Err(Error::from_errno(ret))
> > +        } else {
> > +            Ok(())
> > +        }
> > +    }
> > +
> > +    /// Enable bus-mastering for this device.
> > +    pub fn set_master(&self) {
> > +        // SAFETY: `self.as_raw` is guaranteed to be a pointer to a valid `struct pci_dev`.
> > +        unsafe { bindings::pci_set_master(self.as_raw()) };
> > +    }
> > +}
> > +
> > +impl AsRef<device::Device> for Device {
> > +    fn as_ref(&self) -> &device::Device {
> > +        &self.0
> > +    }
> > +}
> > -- 
> > 2.46.2
> > 
> > 
>