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

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Message-ID: <20250910190239.GA727765@joelbox2>
Date: Wed, 10 Sep 2025 15:02:39 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
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 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. 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?

thanks,

 - Joel