Re: [PATCH v9 08/10] rust: io: introduce `write_reg` and `LocatedRegister`

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

On Sat Mar 14, 2026 at 10:56 PM JST, Gary Guo wrote:
> On Sat Mar 14, 2026 at 1:06 AM GMT, Alexandre Courbot wrote:
>> Some I/O types, like fixed address registers, carry their location
>> alongside their values. For these types, the regular `Io::write` method
>> can lead into repeating the location information twice: once to provide
>> the location itself, another time to build the value.
>>
>> We are also considering supporting making all register values carry
>> their full location information for convenience and safety.
>>
>> Add a new `Io::write_reg` method that takes a single argument
>> implementing `LocatedRegister`, a trait that decomposes implementors
>> into a `(location, value)` tuple. This allows write operations on fixed
>> offset registers to be done while specifying their name only once.
>>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>> ---
>>  rust/kernel/io.rs          | 70 ++++++++++++++++++++++++++++++++++++++++++++++
>>  rust/kernel/io/register.rs | 35 +++++++++++++++++++++--
>>  2 files changed, 103 insertions(+), 2 deletions(-)
>>
>> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
>> index bfea30a9acdf..24e6b48b6582 100644
>> --- a/rust/kernel/io.rs
>> +++ b/rust/kernel/io.rs
>> @@ -17,6 +17,8 @@
>>  pub use crate::register;
>>  pub use resource::Resource;
>>  
>> +use register::LocatedRegister;
>> +
>>  /// Physical address type.
>>  ///
>>  /// This is a type alias to either `u32` or `u64` depending on the config option
>> @@ -473,6 +475,40 @@ fn try_write<T, L>(&self, location: L, value: T) -> Result
>>          Ok(())
>>      }
>>  
>> +    /// Generic fallible write of a fully-located register value.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// Tuples carrying a location and a value can be used with this method:
>> +    ///
>> +    /// ```no_run
>> +    /// use kernel::io::{
>> +    ///     register,
>> +    ///     Io,
>> +    ///     Mmio,
>> +    /// };
>> +    ///
>> +    /// register! {
>> +    ///     FIFO_OUT(u32) @ 0x100 {}
>> +    /// }
>> +    ///
>> +    /// fn do_write_reg(io: &Mmio) -> Result {
>> +    ///     // `FIFO_OUT` provides us the location of the write operation.
>> +    ///     io.try_write_reg(FIFO_OUT::from(10))
>
> This is not a good example, as I want to make FIFO not generate bitfields in the
> future and so people write
>
>     io.write(FIFO_OUT, 10)
>
> Perhaps replace with any other register example that has bitfields..

What is wrong with the example? It demonstrates how we can do a FIFO
register with the current macro.

I was thinking that we can update this example once we have the right
support, but in the meantime this looks useful to me.