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Message-ID: <CAFRnB2XO4hS7RTeNCzxXc4cSNF-XGsUy4yZuYWjKJ1MSvd0gHw@mail.gmail.com>
Date: Mon, 28 Oct 2024 22:08:46 -0400
From: Alex Gaynor <alex.gaynor@...il.com>
To: Rob Herring <robh@...nel.org>
Cc: Saravana Kannan <saravanak@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Rafael J. Wysocki" <rafael@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>,
Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
Danilo Krummrich <dakr@...nel.org>, Dirk Behme <dirk.behme@...il.com>, devicetree@...r.kernel.org,
linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH RFC 2/3] rust: Add bindings for device properties
On Mon, Oct 28, 2024 at 6:24 PM Rob Herring <robh@...nel.org> wrote:
>
> On Fri, Oct 25, 2024 at 4:12 PM Alex Gaynor <alex.gaynor@...il.com> wrote:
> >
> > On Fri, Oct 25, 2024 at 5:06 PM Rob Herring (Arm) <robh@...nel.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@...nel.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
--
All that is necessary for evil to succeed is for good people to do nothing.
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