linux-kernel - Re: [PATCH] rust: pci: add PCI interrupt allocation and management support

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Message-Id: <DCTA2J6Y2PSC.1B48J5ZHUQCOI@kernel.org>
Date: Mon, 15 Sep 2025 11:48:19 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Joel Fernandes" <joelagnelf@...dia.com>
Cc: <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
 <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <linux-pci@...r.kernel.org>, <acourbot@...dia.com>, "Alistair Popple"
 <apopple@...dia.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" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "David Airlie" <airlied@...il.com>,
 "Simona Vetter" <simona@...ll.ch>, "John Hubbard" <jhubbard@...dia.com>,
 "Timur Tabi" <ttabi@...dia.com>, <joel@...lfernandes.org>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, "Bjorn Helgaas" <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>
Subject: Re: [PATCH] rust: pci: add PCI interrupt allocation and management
 support

On Wed Sep 10, 2025 at 9:02 PM CEST, Joel Fernandes wrote:
> On Wed, Sep 10, 2025 at 02:09:55PM -0400, Joel Fernandes wrote:
> [...] 
>> > > +    /// Allocate IRQ vectors for this PCI device.
>> > > +    ///
>> > > +    /// Allocates between `min_vecs` and `max_vecs` interrupt vectors for the device.
>> > > +    /// The allocation will use MSI-X, MSI, or legacy interrupts based on the `irq_types`
>> > > +    /// parameter and hardware capabilities. When multiple types are specified, the kernel
>> > > +    /// will try them in order of preference: MSI-X first, then MSI, then legacy interrupts.
>> > > +    /// This is called during driver probe.
>> > > +    ///
>> > > +    /// # Arguments
>> > > +    ///
>> > > +    /// * `min_vecs` - Minimum number of vectors required
>> > > +    /// * `max_vecs` - Maximum number of vectors to allocate
>> > > +    /// * `irq_types` - Types of interrupts that can be used
>> > > +    ///
>> > > +    /// # Returns
>> > > +    ///
>> > > +    /// Returns the number of vectors successfully allocated, or an error if the allocation
>> > > +    /// fails or cannot meet the minimum requirement.
>> > > +    ///
>> > > +    /// # Examples
>> > > +    ///
>> > > +    /// ```
>> > > +    /// // Allocate using any available interrupt type in the order mentioned above.
>> > > +    /// let nvecs = dev.alloc_irq_vectors(1, 32, IrqTypes::all())?;
>> > > +    ///
>> > > +    /// // Allocate MSI or MSI-X only (no legacy interrupts)
>> > > +    /// let msi_only = IrqTypes::default()
>> > > +    ///     .with(IrqType::Msi)
>> > > +    ///     .with(IrqType::MsiX);
>> > > +    /// let nvecs = dev.alloc_irq_vectors(4, 16, msi_only)?;
>> > > +    /// ```
>> > > +    pub fn alloc_irq_vectors(
>> > > +        &self,
>> > > +        min_vecs: u32,
>> > > +        max_vecs: u32,
>> > > +        irq_types: IrqTypes,
>> > > +    ) -> Result<u32> {
>> > > +        // SAFETY: `self.as_raw` is guaranteed to be a pointer to a valid `struct pci_dev`.
>> > > +        // `pci_alloc_irq_vectors` internally validates all parameters and returns error codes.
>> > > +        let ret = unsafe {
>> > > +            bindings::pci_alloc_irq_vectors(self.as_raw(), min_vecs, max_vecs, irq_types.raw())
>> > > +        };
>> > > +
>> > > +        to_result(ret)?;
>> > > +        Ok(ret as u32)
>> > > +    }
>> > 
>> > This is only valid to be called from the Core context, as it modifies internal
>> > fields of the inner struct device.
>> 
>> It is called from core context, the diff format confuses.
>> > 
>> > Also, it would be nice if it would return a new type that can serve as argument
>> > for irq_vector(), such that we don't have to rely on random integers.
>> 
>> Makes sense, I will do that.
>> 
> By the way, the "ret" value returned by pci_alloc_irq_vectors() is the number
> of vectors, not the vector index.

Sure, but the vector index passed to pci_irq_vector() must be in the range
defined by the return value of pci_alloc_irq_vectors().

I thought of e.g. Range<pci::IrqVector> as return value. This way you can easily
iterate it and prove that it's an allocated vector index.

> So basically there are 3 numbers that mean
> different things:
> 1. Number of vectors (as returned by alloc_irq_vectors).
> 2. Index of a vector (passed to pci_irq_vector).
> 3. The Linux IRQ number (passed to request_irq).
>
> And your point is well taken, in fact even in current code there is
> ambiguity: irq_vector() accepts a vector index, where as request_irq()
> accepts a Linux IRQ number, which are different numbers. I can try to clean
> that up as well but let me know if you had any other thoughts. In fact, I
> think Device<device::Bound>::request_irq() pci should just accept IrqRequest?

Currently, pci::Device::request_irq() takes an IRQ vector index and calls
irq_vector() internally to convert the vector index into an IRQ number.

I'd keep this semantics, but introduce a new type IrqVector rather than using
the raw integer. So, drivers would call

	// `irq_vecs` is of type `Range<pci::IrqVector>`.
	let irq_vecs = dev.alloc_irq_vectors(1, 1, pci::IrqTypes::ANY)?;
	let irq = KBox::pin_init(
	   dev.request_irq(irq_vecs.start, ...)?,
	)?;

Alternatively, to request all of them, if we have multiple, we can leverage
KBox::pin_slice(), which will land in v6.18 (see alloc-next or rust-next branch
in the Rust tree), so all irq::Registration objects can be stored in a single
allocation.