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

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Message-Id: <DDXVGH53P7OW.1UWNG3SM9S0A1@nvidia.com>
Date: Sun, 02 Nov 2025 12:00:33 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Dirk Behme" <dirk.behme@...il.com>, "Alexandre Courbot"
 <acourbot@...dia.com>, "Danilo Krummrich" <dakr@...nel.org>, "Joel
 Fernandes" <joelagnelf@...dia.com>
Cc: <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>,
 "dri-devel" <dri-devel-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH v6 4/5] rust: Move register and bitfield macros out of
 Nova

On Sun Nov 2, 2025 at 3:51 AM JST, Dirk Behme wrote:
> 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> @ ....

Errr indeed, you are correct. The `register` macro's syntax has not been
updated to take advantage of `bitfield`'s storage types, and `u32` is
still hardcoded as of this series.

This looks like an oversight - a register is basically a bitfield with
some I/O, so making it support storage types should be trivial. I guess
this hasn't been done yet because Nova is the only user so far, and we
don't need/want to explicitly specify a type for each register since
they are invariably `u32`.

But it wouldn't look good to change the syntax of `register` after
moving it out, so I agree this should take place before the move. The
same applies to the visiblity feature.

One way to avoid a update all the declarations so far would be to give
Nova its own `register` macro that invokes the one in `kernel` with
the relevant parameters hardcoded.