linux-kernel - Re: [PATCH v6 4/5] rust: Move register and bitfield macros out of Nova

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Message-ID: <7dd6c190-2598-4a68-8431-e03e41b276ea@gmail.com>
Date: Sat, 1 Nov 2025 19:51:16 +0100
From: Dirk Behme <dirk.behme@...il.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 Alistair Popple <apopple@...dia.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, bjorn3_gh@...tonmail.com,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 John Hubbard <jhubbard@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 joel@...lfernandes.org, Elle Rhumsaa <elle@...thered-steel.dev>,
 Yury Norov <yury.norov@...il.com>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 Andrea Righi <arighi@...dia.com>, nouveau@...ts.freedesktop.org,
 Dirk Behme <dirk.behme@...bosch.com>
Subject: Re: [PATCH v6 4/5] rust: Move register and bitfield macros out of
 Nova

On 09.10.25 13:28, Alexandre Courbot wrote:
> On Thu Oct 9, 2025 at 8:16 PM JST, Danilo Krummrich wrote:
>> On Thu Oct 9, 2025 at 8:59 AM CEST, Dirk Behme wrote:
>>> Assuming that register.rs is supposed to become the "generic" way to 
>>> access hardware registers I started to have a look to it. Some weeks 
>>> back testing interrupts I used some quite simple timer with 4 registers 
>>> [1]. Now, thinking about converting it to register!() I have three 
>>> understanding / usage questions:
>>>
>>> * At the moment register!() is for 32-bit registers, only? So it can't 
>>> be used for my example having 8-bit and 16-bit registers as well?
>>
>> Yes, currently the register!() macro always generates a 32-bit register type
>> (mainly because nova-core did not need anything else). However, this will of
>> course be generalized (which should be pretty straight forward).
>>
>> Having a brief look at the TMU datasheet it looks like you should be able to
>> treat TSTR and TCR as 32-bit registers without any issues for testing the
>> register!() macro today. I.e. you can just define it as:
>>
>> 	register!(TSTR @ 0x04, "Timer Start Register" {
>> 	    2:2    str2 as bool, "Specifies whether TCNT2 is operated or stopped.";
>> 	    1:1    str1 as bool, "Specifies whether TCNT1 is operated or stopped.";
>> 	    0:0    str0 as bool, "Specifies whether TCNT0 is operated or stopped.";
>> 	});
>>
>> Same for TCR.
> 
> Patch 2 of this series actually adds support for 16 and 8 bit register
> storage.

Hmm, how to use that with the register!() macro? I mean patch 2 adds
support for different storage widths for *bitfields*. But looking at
patch 4 [2] it looks like *register!()* still uses $name(u32)? With
that it looks like that the register!() macro still just supports 32
bit registers? Or what have I missed?

What I'm looking for is a way to specify if a register is 8, 16 or 32
bit. Using the example from above something like

register!(TSTR<u8> @ ....

Thanks

Dirk

[2]

https://lore.kernel.org/rust-for-linux/20251003154748.1687160-5-joelagnelf@nvidia.com/

...
+#[macro_export]
 macro_rules! register {
     // Creates a register at a fixed offset of the MMIO space.
     ($name:ident @ $offset:literal $(, $comment:literal)? {
$($fields:tt)* } ) => {
-        bitfield!(pub(crate) struct $name(u32) $(, $comment)? {
$($fields)* } );
+        ::kernel::bitfield!(pub(crate) struct $name(u32) $(,
$comment)? { $($fields)* } );
         register!(@io_fixed $name @ $offset);
     };
...