Re: [PATCH] rust: io: always inline methods leading to build_assert

["Alexandre Courbot" <acourbot@nvidia.com>]

On Thu Nov 27, 2025 at 11:53 PM JST, Daniel Almeida wrote:
>
>
>> On 27 Nov 2025, at 10:30, Alexandre Courbot <acourbot@nvidia.com> wrote:
>> 
>> `build_assert` relies on the compiler to optimize out its error path,
>> lest build fails with the dreaded error:
>> 
>>    ERROR: modpost: "rust_build_error" [drivers/gpu/nova-core/nova_core.ko] undefined!
>> 
>> It has been observed that very trivial code performing I/O accesses
>> (sometimes even using an immediate value) would seemingly randomly fail
>> with this error whenever `CLIPPY=1` was set. Removing the CLIPPY option
>> makes the error go away, but that's obviously not a great workaround.
>> Clippy appears to influence the way the compiler optimizes things,
>> making it on occasion generate a method where we would need it to inline
>> in order to satisfy a `build_assert`.
>> 
>> Fix this by instructing the compiler to always inline the methods
>> leading to `build_assert`. This stronger directive is effective even
>> when `CLIPPY=1` is specified, which gets rid of this error.
>> 
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>> ---
>> This is the same fix as for another build error triggered by the use of
>> `build_assert` [1], which signals that all callers of this macro should
>> all be tagged with `#[inline(always)]`, as inlining is a requirement for
>> `build_assert` to perform properly anyway.
>> 
>> [1] https://lore.kernel.org/all/DEEUYUOAEZU3.1J1HM2YQ10EX1@nvidia.com/
>> ---
>> rust/kernel/io.rs | 9 ++++++---
>> 1 file changed, 6 insertions(+), 3 deletions(-)
>> 
>> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
>> index 98e8b84e68d1..f161ec8056ce 100644
>> --- a/rust/kernel/io.rs
>> +++ b/rust/kernel/io.rs
>> @@ -142,7 +142,8 @@ macro_rules! define_read {
>>         /// Bound checks are performed on compile time, hence if the offset is not known at compile
>>         /// time, the build will fail.
>>         $(#[$attr])*
>> -        #[inline]
>> +        // Always inline so the error path of `io_addr_assert` is optimized out.
>> +        #[inline(always)]
>>         pub fn $name(&self, offset: usize) -> $type_name {
>>             let addr = self.io_addr_assert::<$type_name>(offset);
>> 
>> @@ -171,7 +172,8 @@ macro_rules! define_write {
>>         /// Bound checks are performed on compile time, hence if the offset is not known at compile
>>         /// time, the build will fail.
>>         $(#[$attr])*
>> -        #[inline]
>> +        // Always inline so the error path of `io_addr_assert` is optimized out.
>> +        #[inline(always)]
>>         pub fn $name(&self, value: $type_name, offset: usize) {
>>             let addr = self.io_addr_assert::<$type_name>(offset);
>> 
>> @@ -239,7 +241,8 @@ fn io_addr<U>(&self, offset: usize) -> Result<usize> {
>>         self.addr().checked_add(offset).ok_or(EINVAL)
>>     }
>> 
>> -    #[inline]
>> +    // Always inline so the error path of `build_assert!` is optimized out.
>> +    #[inline(always)]
>>     fn io_addr_assert<U>(&self, offset: usize) -> usize {
>>         build_assert!(Self::offset_valid::<U>(offset, SIZE));
>> 
>> 
>> ---
>> base-commit: ea34511aaf755349999a1067b2984a541bee1492
>> change-id: 20251127-io-build-assert-3579a5bfb81c
>> 
>> Best regards,
>> -- 
>> Alexandre Courbot <acourbot@nvidia.com>
>> 
>> 
>
> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
>
> I also faced this with genmask, by the way, i.e.: using genmask with an
> in-bounds constant would trigger a build error. Very confusingly, this would be
> randomly solved by moving the genmask invocation around in the code.
>
> I wonder if the same fix is needed for it as well?

What you described is exactly the symptoms I was experimenting (notably
the "moving around sometimes fixes it" thing) so yeah, I think this also
applies to `bit_*` and `genmask_*` (and anything that invokes
`build_assert`, really).