Re: [PATCH v1] rust: refactor `to_result` to return the original value

[Daniel Almeida <daniel.almeida@collabora.com>]

> On 9 Sep 2025, at 09:36, Alice Ryhl <aliceryhl@google.com> wrote:
> 
> On Tue, Sep 9, 2025 at 2:33 PM Onur Özkan <work@onurozkan.dev> wrote:
>> 
>> Current `to_result` helper takes a `c_int` and returns `Ok(())` on
>> success and this has some issues like:
>> 
>>    - Callers lose the original return value and often have to store
>>        it in a temporary variable before calling `to_result`.
>> 
>>    - It only supports `c_int`, which makes callers to unnecessarily
>>        cast when working with other types (e.g. `u16` in phy
>>        abstractions). We even have some places that ignore to use
>>        `to_result` helper because the input doesn't fit in `c_int`
>>        (see [0]).
>> 
>> [0]: https://lore.kernel.org/all/20250822080252.773d6f54@nimda.home/
>> 
>> This patch changes `to_result` to be generic and also return the
>> original value on success.
>> 
>> So that the code that previously looked like:
>> 
>>    let ret = unsafe { bindings::some_ffi_call() };
>>    to_result(ret).map(|()| SomeType::new(ret))
>> 
>> can now be written more directly as:
>> 
>>    to_result(unsafe { bindings::some_ffi_call() })
>>        .map(|ret| SomeType::new(ret))
>> 
>> Similarly, code such as:
>> 
>>    let res: isize = $some_ffi_call();
>>    if res < 0 {
>>        return Err(Error::from_errno(res as i32));
>>    }
>> 
>> can now be used with `to_result` as:
>> 
>>    to_result($some_ffi_call())?;
>> 
>> Existing call sites that only care about success/failure are updated
>> to append `.map(|_| ())` to preserve their previous semantics. They
>> can also use the equivalent pattern:
>> 
>>    to_result($something)?;
>>    Ok(())
>> 
>> This patch only fixes the callers that broke after the changes on `to_result`.

What exactly broke for regulator.rs <http://regulator.rs/>? It works just fine.

>> I haven't included all the improvements made possible by the new design since
>> that could conflict with other ongoing patches [1]. Once this patch is approved
>> and applied, I am planning to follow up with creating a "good first issue" on [2]
>> for those additional changes.
>> 
>> [1]: https://lore.kernel.org/rust-for-linux/?q=to_result
>> [2]: https://github.com/Rust-for-Linux/linux
>> 
>> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089/topic/x/near/536374456
>> Signed-off-by: Onur Özkan <work@onurozkan.dev>
>> ---
>> rust/kernel/auxiliary.rs        |  1 +
>> rust/kernel/block/mq/tag_set.rs |  2 +-
>> rust/kernel/cpufreq.rs          |  3 ++-
>> rust/kernel/devres.rs           |  1 +
>> rust/kernel/dma.rs              |  3 +++
>> rust/kernel/error.rs            | 17 ++++++++++++-----
>> rust/kernel/miscdevice.rs       |  2 +-
>> rust/kernel/mm/virt.rs          |  1 +
>> rust/kernel/pci.rs              |  3 ++-
>> rust/kernel/platform.rs         |  2 +-
>> rust/kernel/regulator.rs        |  5 +++--
>> 11 files changed, 28 insertions(+), 12 deletions(-)
>> 
>> diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
>> index 4749fb6bffef..479c0ad2a572 100644
>> --- a/rust/kernel/auxiliary.rs
>> +++ b/rust/kernel/auxiliary.rs
>> @@ -43,6 +43,7 @@ unsafe fn register(
>>         to_result(unsafe {
>>             bindings::__auxiliary_driver_register(adrv.get(), module.0, name.as_char_ptr())
>>         })
>> +        .map(|_| ())
>>     }
>> 
>>     unsafe fn unregister(adrv: &Opaque<Self::RegType>) {
>> diff --git a/rust/kernel/block/mq/tag_set.rs b/rust/kernel/block/mq/tag_set.rs
>> index c3cf56d52bee..0e7883163000 100644
>> --- a/rust/kernel/block/mq/tag_set.rs
>> +++ b/rust/kernel/block/mq/tag_set.rs
>> @@ -65,7 +65,7 @@ pub fn new(
>>                 // SAFETY: we do not move out of `tag_set`.
>>                 let tag_set: &mut Opaque<_> = unsafe { Pin::get_unchecked_mut(tag_set) };
>>                 // SAFETY: `tag_set` is a reference to an initialized `blk_mq_tag_set`.
>> -                error::to_result( unsafe { bindings::blk_mq_alloc_tag_set(tag_set.get())})
>> +                error::to_result( unsafe { bindings::blk_mq_alloc_tag_set(tag_set.get())}).map(|_| ())
>>             }),
>>             _p: PhantomData,
>>         })
>> diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
>> index be2dffbdb546..c3fa20ce229a 100644
>> --- a/rust/kernel/cpufreq.rs
>> +++ b/rust/kernel/cpufreq.rs
>> @@ -157,6 +157,7 @@ pub fn as_raw(&self) -> *mut bindings::cpufreq_policy_data {
>>     pub fn generic_verify(&self) -> Result {
>>         // SAFETY: By the type invariant, the pointer stored in `self` is valid.
>>         to_result(unsafe { bindings::cpufreq_generic_frequency_table_verify(self.as_raw()) })
>> +            .map(|_| ())
>>     }
>> }
>> 
>> @@ -519,7 +520,7 @@ pub fn set_suspend_freq(&mut self, freq: Hertz) -> &mut Self {
>>     #[inline]
>>     pub fn generic_suspend(&mut self) -> Result {
>>         // SAFETY: By the type invariant, the pointer stored in `self` is valid.
>> -        to_result(unsafe { bindings::cpufreq_generic_suspend(self.as_mut_ref()) })
>> +        to_result(unsafe { bindings::cpufreq_generic_suspend(self.as_mut_ref()) }).map(|_| ())
>>     }
>> 
>>     /// Provides a wrapper to the generic get routine.
>> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
>> index d04e3fcebafb..214cd9a0ebe0 100644
>> --- a/rust/kernel/devres.rs
>> +++ b/rust/kernel/devres.rs
>> @@ -328,6 +328,7 @@ fn register_foreign<P>(dev: &Device<Bound>, data: P) -> Result
>>         // `ForeignOwnable` is released eventually.
>>         bindings::devm_add_action_or_reset(dev.as_raw(), Some(callback::<P>), ptr.cast())
>>     })
>> +    .map(|_| ())
>> }
>> 
>> /// Encapsulate `data` in a [`KBox`] and [`Drop::drop`] `data` once `dev` is unbound.
>> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
>> index 68fe67624424..f614453ddb7d 100644
>> --- a/rust/kernel/dma.rs
>> +++ b/rust/kernel/dma.rs
>> @@ -34,6 +34,7 @@ unsafe fn dma_set_mask(&self, mask: DmaMask) -> Result {
>>         // - The safety requirement of this function guarantees that there are no concurrent calls
>>         //   to DMA allocation and mapping primitives using this mask.
>>         to_result(unsafe { bindings::dma_set_mask(self.as_ref().as_raw(), mask.value()) })
>> +            .map(|_| ())
>>     }
>> 
>>     /// Set up the device's DMA coherent addressing capabilities.
>> @@ -51,6 +52,7 @@ unsafe fn dma_set_coherent_mask(&self, mask: DmaMask) -> Result {
>>         // - The safety requirement of this function guarantees that there are no concurrent calls
>>         //   to DMA allocation and mapping primitives using this mask.
>>         to_result(unsafe { bindings::dma_set_coherent_mask(self.as_ref().as_raw(), mask.value()) })
>> +            .map(|_| ())
>>     }
>> 
>>     /// Set up the device's DMA addressing capabilities.
>> @@ -72,6 +74,7 @@ unsafe fn dma_set_mask_and_coherent(&self, mask: DmaMask) -> Result {
>>         to_result(unsafe {
>>             bindings::dma_set_mask_and_coherent(self.as_ref().as_raw(), mask.value())
>>         })
>> +        .map(|_| ())
>>     }
>> }
>> 
>> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
>> index db14da976722..f76afa4b7ec1 100644
>> --- a/rust/kernel/error.rs
>> +++ b/rust/kernel/error.rs
>> @@ -378,12 +378,19 @@ fn from(e: core::convert::Infallible) -> Error {
>> pub type Result<T = (), E = Error> = core::result::Result<T, E>;
>> 
>> /// Converts an integer as returned by a C kernel function to an error if it's negative, and
>> -/// `Ok(())` otherwise.
>> -pub fn to_result(err: crate::ffi::c_int) -> Result {
>> -    if err < 0 {
>> -        Err(Error::from_errno(err))
>> +/// returns the original value otherwise.
>> +pub fn to_result<T>(code: T) -> Result<T>
>> +where
>> +    T: Copy + TryInto<i32>,
>> +{
>> +    // Try casting into `i32`.
>> +    let casted: crate::ffi::c_int = code.try_into().unwrap_or(0);
>> +
>> +    if casted < 0 {
>> +        Err(Error::from_errno(casted))
>>     } else {
>> -        Ok(())
>> +        // Return the original input value.
>> +        Ok(code)
>>     }
>> }
>> 
>> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
>> index 6373fe183b27..22b72ae84c03 100644
>> --- a/rust/kernel/miscdevice.rs
>> +++ b/rust/kernel/miscdevice.rs
>> @@ -79,7 +79,7 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
>>                 // the destructor of this type deallocates the memory.
>>                 // INVARIANT: If this returns `Ok(())`, then the `slot` will contain a registered
>>                 // misc device.
>> -                to_result(unsafe { bindings::misc_register(slot) })
>> +                to_result(unsafe { bindings::misc_register(slot) }).map(|_| ())
>>             }),
>>             _t: PhantomData,
>>         })
>> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
>> index a1bfa4e19293..5494f96e91b0 100644
>> --- a/rust/kernel/mm/virt.rs
>> +++ b/rust/kernel/mm/virt.rs
>> @@ -195,6 +195,7 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>>         // SAFETY: By the type invariant of `Self` caller has read access and has verified that
>>         // `VM_MIXEDMAP` is set. By invariant on `Page` the page has order 0.
>>         to_result(unsafe { bindings::vm_insert_page(self.as_ptr(), address, page.as_ptr()) })
>> +            .map(|_| ())
>>     }
>> }
>> 
>> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
>> index 887ee611b553..6e917752cb89 100644
>> --- a/rust/kernel/pci.rs
>> +++ b/rust/kernel/pci.rs
>> @@ -48,6 +48,7 @@ unsafe fn register(
>>         to_result(unsafe {
>>             bindings::__pci_register_driver(pdrv.get(), module.0, name.as_char_ptr())
>>         })
>> +        .map(|_| ())
>>     }
>> 
>>     unsafe fn unregister(pdrv: &Opaque<Self::RegType>) {
>> @@ -437,7 +438,7 @@ impl Device<device::Core> {
>>     /// Enable memory resources for this device.
>>     pub fn enable_device_mem(&self) -> Result {
>>         // SAFETY: `self.as_raw` is guaranteed to be a pointer to a valid `struct pci_dev`.
>> -        to_result(unsafe { bindings::pci_enable_device_mem(self.as_raw()) })
>> +        to_result(unsafe { bindings::pci_enable_device_mem(self.as_raw()) }).map(|_| ())
>>     }
>> 
>>     /// Enable bus-mastering for this device.
>> diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
>> index 8f028c76f9fa..5a5561c7326e 100644
>> --- a/rust/kernel/platform.rs
>> +++ b/rust/kernel/platform.rs
>> @@ -54,7 +54,7 @@ unsafe fn register(
>>         }
>> 
>>         // SAFETY: `pdrv` is guaranteed to be a valid `RegType`.
>> -        to_result(unsafe { bindings::__platform_driver_register(pdrv.get(), module.0) })
>> +        to_result(unsafe { bindings::__platform_driver_register(pdrv.get(), module.0) }).map(|_| ())
>>     }
>> 
>>     unsafe fn unregister(pdrv: &Opaque<Self::RegType>) {
>> diff --git a/rust/kernel/regulator.rs b/rust/kernel/regulator.rs
>> index 65f3a125348f..e17ae6e9a990 100644
>> --- a/rust/kernel/regulator.rs
>> +++ b/rust/kernel/regulator.rs
>> @@ -261,6 +261,7 @@ pub fn set_voltage(&self, min_voltage: Voltage, max_voltage: Voltage) -> Result
>>                 max_voltage.as_microvolts(),
>>             )
>>         })
>> +        .map(|_| ())
>>     }
>> 
>>     /// Gets the current voltage of the regulator.
>> @@ -291,12 +292,12 @@ fn get_internal(dev: &Device, name: &CStr) -> Result<Regulator<T>> {
>> 
>>     fn enable_internal(&mut self) -> Result {
>>         // SAFETY: Safe as per the type invariants of `Regulator`.
>> -        to_result(unsafe { bindings::regulator_enable(self.inner.as_ptr()) })
>> +        to_result(unsafe { bindings::regulator_enable(self.inner.as_ptr()) }).map(|_| ())
>>     }
>> 
>>     fn disable_internal(&mut self) -> Result {
>>         // SAFETY: Safe as per the type invariants of `Regulator`.
>> -        to_result(unsafe { bindings::regulator_disable(self.inner.as_ptr()) })
>> +        to_result(unsafe { bindings::regulator_disable(self.inner.as_ptr()) }).map(|_| ())
> 
> IMO all of the new map calls in this patch should use the
> 
> to_result(...)?
> Ok(())
> 
> syntax.
> 
> Alice

FYI: A similar patch was already applied by Mark to regulator.rs, but now I see
the same code being submitted a) under a different patch, with its own version
(v1) and b) together as a single patch touching a lot of code at once.

I am a bit confused. Can you make sure that all code touching regulator.rs is
based on the regulator tree? 

— Daniel