linux-kernel - Re: [PATCH] rust/drm: tyr: Convert to the register!() macro

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Message-Id: <DFQ3NOFEAIRC.3V5QZUXJJ7CMA@kernel.org>
Date: Fri, 16 Jan 2026 15:56:44 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Daniel Almeida" <daniel.almeida@...labora.com>
Cc: "Dirk Behme" <Dirk.Behme@...bosch.com>, "Dirk Behme"
 <dirk.behme@...il.com>, "Alice Ryhl" <aliceryhl@...gle.com>, "Maarten
 Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "David
 Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>,
 "Steven Price" <steven.price@....com>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 "Alexandre Courbot" <acourbot@...dia.com>
Subject: Re: [PATCH] rust/drm: tyr: Convert to the register!() macro

On Fri Jan 16, 2026 at 1:26 PM CET, Daniel Almeida wrote:
> +cc Alex,
>
>
>>>> +    31:0    l2_pwractive_hi as u32, "Bitmap of L2 caches active (higher 32 bits)";
>>>> +});
>>>> +
>>>>  pub(crate) const MCU_CONTROL_ENABLE: u32 = 1;
>>>>  pub(crate) const MCU_CONTROL_AUTO: u32 = 2;
>>>>  pub(crate) const MCU_CONTROL_DISABLE: u32 = 0;
>>>>  -pub(crate) const MCU_STATUS: Register<0x704> = Register;
>>>> +register!(McuControl @ 0x700, "Controls the execution state of the MCU subsystem" {
>>>> +    1:0     req as u32, "Request state change";
>>>> +});
>>> Any reason why req is a u32 and not a u8? Same for some other places.
>>> And would it be an option to move the const MCU_CONTROL* to an ìmpl
>>> McuControl Same for STATUS below.
>> 
>> Just fyi something like [1] builds for me.
>> 
>> This is inspired by
>> 
>> https://lore.kernel.org/rust-for-linux/20251003154748.1687160-6-joelagnelf@nvidia.com/
>> 
>> Best regards
>> 
>> Dirk
>> 
>> [1]
>> 
>> #[repr(u32)]
>> #[derive(Debug, Default, Clone, Copy, PartialEq)]
>> enum McuControl {
>>    #[default]
>>    Disable = 0,
>>    Enable = 1,
>>    Auto = 2,
>> }
>> 
>> impl From<McuControl> for u8 {
>>    fn from(ctrl: McuControl) -> Self {
>>        ctrl as u8
>>    }
>> }
>> 
>> impl From<u8> for McuControl {
>>    fn from(req: u8) -> Self {
>>        match req & 0x3 {
>>            0 => McuControl::Disable,
>>            1 => McuControl::Enable,
>>            2 => McuControl::Auto,
>>            _ => McuControl::Disable,
>>        }
>>    }
>> }
>> 
>> register!(MCUCONTROL @ 0x700, "Controls the execution state of the MCU subsystem" {
>>    1:0     req as u8 => McuControl, "Request state change";
>> });
>
> Alex, looking at the above, I wonder if a “as Foo” would be a good
> addition to the macro? That would then invoke a TryFrom implementation, i.e.:
>
>
> register!(MCUCONTROL @ 0x700, "Controls the execution state of the MCU subsystem" {
>    1:0     req as McuControl => McuControl, "Request state change";
> });

This would imply the assumption that req is treated as u8 by register!()
automatically before calling the TryFrom impl.

One could argue that this is reasonable, since the value is only two bits wide,
but it might not always be desired. I think keeping this explict is better.