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

[Joel Fernandes <joelagnelf@nvidia.com>]

On Mon, Sep 15, 2025 at 11:48:19AM +0200, Danilo Krummrich wrote:
> 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.

Agreed, I will do it like this.

> > 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, ...)?,
> 	)?;

This sounds good to me. Thanks,

 - Joel