Re: [PATCH v2 2/2] rust: add firmware abstractions

[Danilo Krummrich]

On Tue, Jun 11, 2024 at 08:31:46AM +0200, Greg KH wrote:
> On Mon, Jun 10, 2024 at 08:02:28PM +0200, Danilo Krummrich wrote:
> > Add an abstraction around the kernels firmware API to request firmware
> > images. The abstraction provides functions to access the firmware's size
> > and backing buffer.
> > 
> > The firmware is released once the abstraction instance is dropped.
> > 
> > Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> > ---
> >  rust/bindings/bindings_helper.h |   1 +
> >  rust/kernel/firmware.rs         | 107 ++++++++++++++++++++++++++++++++
> >  rust/kernel/lib.rs              |   1 +
> >  3 files changed, 109 insertions(+)
> >  create mode 100644 rust/kernel/firmware.rs
> > 
> > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> > index ddb5644d4fd9..18a3f05115cb 100644
> > --- a/rust/bindings/bindings_helper.h
> > +++ b/rust/bindings/bindings_helper.h
> > @@ -9,6 +9,7 @@
> >  #include <kunit/test.h>
> >  #include <linux/errname.h>
> >  #include <linux/ethtool.h>
> > +#include <linux/firmware.h>
> >  #include <linux/jiffies.h>
> >  #include <linux/mdio.h>
> >  #include <linux/phy.h>
> > diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> > new file mode 100644
> > index 000000000000..7ff4c325f670
> > --- /dev/null
> > +++ b/rust/kernel/firmware.rs
> > @@ -0,0 +1,107 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Firmware abstraction
> > +//!
> > +//! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h")
> > +
> > +use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
> > +use core::ptr::NonNull;
> > +
> > +// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
> > +// `firmware_request_platform`, `bindings::request_firmware_direct`
> > +type FwFunc =
> > +    unsafe extern "C" fn(*mut *const bindings::firmware, *const i8, *mut bindings::device) -> i32;
> > +
> > +/// Abstraction around a C `struct firmware`.
> > +///
> > +/// This is a simple abstraction around the C firmware API. Just like with the C API, firmware can
> > +/// be requested. Once requested the abstraction provides direct access to the firmware buffer as
> > +/// `&[u8]`. The firmware is released once [`Firmware`] is dropped.
> > +///
> > +/// # Invariants
> > +///
> > +/// The pointer is valid, and has ownership over the instance of `struct firmware`.
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// use kernel::firmware::Firmware;
> > +///
> > +/// let fw = Firmware::request("path/to/firmware.bin", dev.as_ref())?;
> > +/// driver_load_firmware(fw.data());
> > +/// ```
> > +pub struct Firmware(NonNull<bindings::firmware>);
> > +
> > +impl Firmware {
> > +    fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
> > +        let mut fw: *mut bindings::firmware = core::ptr::null_mut();
> > +        let pfw: *mut *mut bindings::firmware = &mut fw;
> > +
> > +        // SAFETY: `pfw` is a valid pointer to a NULL initialized `bindings::firmware` pointer.
> > +        // `name` and `dev` are valid as by their type invariants.
> > +        let ret = unsafe { func(pfw as _, name.as_char_ptr(), dev.as_raw()) };
> > +        if ret != 0 {
> > +            return Err(Error::from_errno(ret));
> > +        }
> > +
> > +        // SAFETY: `func` not bailing out with a non-zero error code, guarantees that `fw` is a
> > +        // valid pointer to `bindings::firmware`.
> > +        Ok(Firmware(unsafe { NonNull::new_unchecked(fw) }))
> > +    }
> > +
> > +    /// Send a firmware request and wait for it. See also `bindings::request_firmware`.
> > +    pub fn request(name: &CStr, dev: &Device) -> Result<Self> {
> > +        Self::request_internal(name, dev, bindings::request_firmware)
> > +    }
> 
> How does this handle when CONFIG_FW_LOADER is not enabled?  Why are you
> building these bindings if that option is not checked?

Good catch, gonna fix it.

> 
> > +
> > +    /// Send a request for an optional firmware module. See also
> > +    /// `bindings::firmware_request_nowarn`.
> > +    pub fn request_nowarn(name: &CStr, dev: &Device) -> Result<Self> {
> > +        Self::request_internal(name, dev, bindings::firmware_request_nowarn)
> > +    }
> > +
> > +    /// Send a request for a firmware with platform-fw fallback. See also
> > +    /// `bindings::firmware_request_platform`.
> > +    pub fn request_platform(name: &CStr, dev: &Device) -> Result<Self> {
> > +        Self::request_internal(name, dev, bindings::firmware_request_platform)
> > +    }
> > +
> > +    /// Send a request for a firmware directly without usermode helper. See also
> > +    /// `bindings::request_firmware_direct`.
> > +    pub fn request_direct(name: &CStr, dev: &Device) -> Result<Self> {
> > +        Self::request_internal(name, dev, bindings::request_firmware_direct)
> > +    }
> 
> Why just these variants?  Why not just add the ones that people actually
> need instead of a random assortment like you choose here :)

Indeed seems a bit random, not entirely though. I chose `request_firmware` and
`firmware_request_nowarn` since those are the ones we need in Nova, maybe we can
switch some calls to `request_firmware_into_buf` in the future, once we got the
allocator API in place.

I added `firmware_request_platform` and `request_firmware_direct` as well, since
they share the same function signature as the ones mentioned above and hence all
four of them share the same implementation through `Firmware::request_internal`,
just passing a different function pointer.

If you prefer I can drop the latter for now though.

- Danilo

> 
> thanks,
> 
> greg k-h
>