Re: [PATCH RFC 2/3] rust: Add bindings for device properties

[Alex Gaynor <alex.gaynor@gmail.com>]

On Mon, Oct 28, 2024 at 6:24 PM Rob Herring <robh@kernel.org> wrote:
>
> On Fri, Oct 25, 2024 at 4:12 PM Alex Gaynor <alex.gaynor@gmail.com> wrote:
> >
> > On Fri, Oct 25, 2024 at 5:06 PM Rob Herring (Arm) <robh@kernel.org> wrote:
> > >
> > > The device property API is a firmware agnostic API for reading
> > > properties from firmware (DT/ACPI) devices nodes and swnodes.
> > >
> > > While the C API takes a pointer to a caller allocated variable/buffer,
> > > the rust API is designed to return a value and can be used in struct
> > > initialization. Rust generics are also utilized to support different
> > > sizes of properties (e.g. u8, u16, u32).
> > >
> > > Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
> > > ---
> > > Not sure if we need the KVec variant, but I kept it as that was my first
> > > pass attempt. Most callers are filling in some value in a driver data
> > > struct. Sometimes the number of elements is not known, so the caller
> > > calls to get the array size, allocs the correct size buffer, and then
> > > reads the property again to fill in the buffer.
> > >
> > > I have not implemented a wrapper for device_property_read_string(_array)
> > > because that API is problematic for dynamic DT nodes. The API just
> > > returns pointer(s) into the raw DT data. We probably need to return a
> > > copy of the string(s) instead for rust.
> > >
> > > After property accessors, next up is child node accessors/iterators.
> > > ---
> > >  rust/bindings/bindings_helper.h |   1 +
> > >  rust/kernel/device.rs           | 145 +++++++++++++++++++++++++++++++++++++++-
> > >  2 files changed, 145 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> > > index 217c776615b9..65717cc20a23 100644
> > > --- a/rust/bindings/bindings_helper.h
> > > +++ b/rust/bindings/bindings_helper.h
> > > @@ -19,6 +19,7 @@
> > >  #include <linux/pci.h>
> > >  #include <linux/phy.h>
> > >  #include <linux/platform_device.h>
> > > +#include <linux/property.h>
> > >  #include <linux/refcount.h>
> > >  #include <linux/sched.h>
> > >  #include <linux/slab.h>
> > > diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> > > index 0c28b1e6b004..bb66a28df890 100644
> > > --- a/rust/kernel/device.rs
> > > +++ b/rust/kernel/device.rs
> > > @@ -5,10 +5,14 @@
> > >  //! C header: [`include/linux/device.h`](srctree/include/linux/device.h)
> > >
> > >  use crate::{
> > > +    alloc::KVec,
> > >      bindings,
> > > +    error::{to_result, Result},
> > > +    prelude::*,
> > > +    str::CStr,
> > >      types::{ARef, Opaque},
> > >  };
> > > -use core::{fmt, ptr};
> > > +use core::{fmt, mem::size_of, ptr};
> > >
> > >  #[cfg(CONFIG_PRINTK)]
> > >  use crate::c_str;
> > > @@ -189,6 +193,145 @@ unsafe fn printk(&self, klevel: &[u8], msg: fmt::Arguments<'_>) {
> > >              )
> > >          };
> > >      }
> > > +
> > > +    /// Returns if a firmware property `name` is true or false
> > > +    pub fn property_read_bool(&self, name: &CStr) -> bool {
> > > +        unsafe { bindings::device_property_present(self.as_raw(), name.as_ptr() as *const i8) }
> > > +    }
> > > +
> > > +    /// Returns if a firmware string property `name` has match for `match_str`
> > > +    pub fn property_match_string(&self, name: &CStr, match_str: &CStr) -> Result<usize> {
> > > +        let ret = unsafe {
> > > +            bindings::device_property_match_string(
> > > +                self.as_raw(),
> > > +                name.as_ptr() as *const i8,
> > > +                match_str.as_ptr() as *const i8,
> > > +            )
> > > +        };
> > > +        to_result(ret)?;
> > > +        Ok(ret as usize)
> > > +    }
> > > +
> > > +    /// Returns firmware property `name` scalar value
> > > +    ///
> > > +    /// Valid types are i8, u8, i16, u16, i32, u32, i64, u64
> > > +    pub fn property_read<T: Copy>(&self, name: &CStr) -> Result<T> {
> > > +        let mut val: [T; 1] = unsafe { core::mem::zeroed() };
> > > +
> > > +        Self::_property_read_array(&self, name, &mut val)?;
> > > +        Ok(val[0])
> > > +    }
> > > +
> >
> > This, and several of the other methods are unsound, because they can
> > be used to construct arbitrary types for which may not allow all bit
> > patterns. You can use:
> > https://rust.docs.kernel.org/kernel/types/trait.FromBytes.html as the
> > bound to ensure only valid types are used.
> >
> > Also, instead of using mem::zeroed(), you should use MaybeUnininit
> > (https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html)
> > which allows you to avoid needing to zero initialize.
>
> Something like this what you had in mind?:
>
> pub fn property_read_array<T, const N: usize>(&self, name: &CStr) ->
> Result<[T; N]> {
>     let mut val: [MaybeUninit<T>; N] = [const { MaybeUninit::uninit() }; N];
>
>     Self::_property_read_array(self, name, &mut val)?;
>
>     // SAFETY: On success, _property_read_array has filled in the array
>     let val = unsafe { mem::transmute_copy(&val) };
>     Ok(val)
> }

Yes, that's right. Ideally you could use
https://doc.rust-lang.org/stable/core/mem/union.MaybeUninit.html#method.array_assume_init
instead of transmute, but it's not yet stable.

Alex

-- 
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