Re: [PATCH v8 1/1] rust: pci: add extended capability and SR-IOV support

[Alexandre Courbot]

On Tue Aug 18, 2026 at 5:46 PM JST, Zhi Wang wrote:
> Rust PCI drivers have no typed interface for locating and accessing PCIe
> extended capabilities.
>
> The SR-IOV extended capability describes VF topology and VF BARs. Expose
> this information through the Rust PCI abstraction so drivers can use the
> existing typed configuration-space accessors instead of raw bindings.
>
> Define ExtCapability to associate a capability ID with a register layout,
> and add ConfigSpace::find_ext_capability() to locate and project that
> layout. Bound the view at the next capability or the end of extended
> configuration space. Add ExtSriovRegs and a decoded VF BAR iterator that
> reads and validates all six VF BAR register slots up front, yields decoded
> BAR addresses and widths in logical order, and keeps the raw
> configuration-space slot advancement internal. Since PCI_EXT_CAP_NEXT() is
> a function-like macro, expose it through a Rust helper.
>
> Link: https://lore.kernel.org/rust-for-linux/20260804161612.776752-1-zhiw@xxxxxxxxxx/
> Cc: Alexandre Courbot <acourbot@xxxxxxxxxx>
> Cc: Gary Guo <gary@xxxxxxxxxxx>
> Signed-off-by: Zhi Wang <zhiw@xxxxxxxxxx>
> ---
>  rust/helpers/pci.c     |   5 +
>  rust/kernel/pci.rs     |   8 +
>  rust/kernel/pci/cap.rs | 329 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 342 insertions(+)
>  create mode 100644 rust/kernel/pci/cap.rs
>
> diff --git a/rust/helpers/pci.c b/rust/helpers/pci.c
> index 4ebf256dff23..b946b14d79e4 100644
> --- a/rust/helpers/pci.c
> +++ b/rust/helpers/pci.c
> @@ -24,6 +24,11 @@ __rust_helper bool rust_helper_dev_is_pci(const struct device *dev)
>  	return dev_is_pci(dev);
>  }
>  
> +__rust_helper u32 rust_helper_pci_ext_cap_next(u32 header)
> +{
> +	return PCI_EXT_CAP_NEXT(header);
> +}
> +
>  #ifndef CONFIG_PCI_IOV
>  __rust_helper unsigned int
>  rust_helper_pci_sriov_get_totalvfs(struct pci_dev *pdev)
> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> index 9f19ccd5905c..008c2770a3f3 100644
> --- a/rust/kernel/pci.rs
> +++ b/rust/kernel/pci.rs
> @@ -32,10 +32,18 @@
>      },
>  };
>  
> +mod cap;
>  mod id;
>  mod io;
>  mod irq;
>  
> +pub use self::cap::{
> +    ExtCapId,
> +    ExtCapability,
> +    ExtSriovCapability,
> +    ExtSriovRegs,
> +    ExtSriovVfBar, //
> +};
>  pub use self::id::{
>      Class,
>      ClassMask,
> diff --git a/rust/kernel/pci/cap.rs b/rust/kernel/pci/cap.rs
> new file mode 100644
> index 000000000000..ddb3fd73e195
> --- /dev/null
> +++ b/rust/kernel/pci/cap.rs
> @@ -0,0 +1,329 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! PCI extended capability support.
> +
> +use super::{
> +    io::ConfigSpaceBackend,
> +    ConfigSpace,
> +    Extended, //
> +};
> +use crate::{
> +    bindings,
> +    io::{
> +        Io,
> +        IoBackend,
> +        Region, //
> +    },
> +    num::Bounded,
> +    prelude::*,
> +};
> +
> +/// Number of VF BAR register slots in an SR-IOV capability.
> +// CAST: `PCI_SRIOV_NUM_BARS` is the PCIe-specified number of VF BAR register slots and fits in
> +// `usize`.
> +const NUM_VF_BARS: usize = bindings::PCI_SRIOV_NUM_BARS as usize;
> +
> +/// PCI extended capability IDs.
> +#[repr(transparent)]
> +#[derive(Debug, Clone, Copy, PartialEq, Eq)]
> +pub struct ExtCapId(u16);
> +
> +impl ExtCapId {
> +    /// Single Root I/O Virtualization.
> +    // CAST: PCI extended capability IDs are 16-bit values defined by the PCIe specification.
> +    pub const SRIOV: Self = Self(bindings::PCI_EXT_CAP_ID_SRIOV as u16);

Coming back to comment on the conclusion: we have settled on a solution
[1] to handle this nicely, but it is likely that your series will land
before it. So meanwhile, please use the turbofish solution that I
initially proposed:

    pub const SRIOV: Self = Self(casts::u32_into_u16::<{ bindings::PCI_EXT_CAP_ID_SRIOV }>());

While it's a mouthful, [1] does remove `u32_into_u16`, which means we
won't miss it and will need to update the code to use `const_as!` when
applying.

[1] https://lore.kernel.org/all/20260825-const_as-v1-1-1ce712225fe2@xxxxxxxxxx/