linux-kernel - Re: [PATCH v7.1 2/4] gpu: nova-core: bitfield: Move bitfield-specific code from register! into new macro

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Message-Id: <DDNIZMOBCE1Z.1ZOQ83TKTHQ28@kernel.org>
Date: Tue, 21 Oct 2025 01:07:40 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "John Hubbard" <jhubbard@...dia.com>
Cc: "Joel Fernandes" <joelagnelf@...dia.com>, "Yury Norov"
 <yury.norov@...il.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "rust-for-linux@...r.kernel.org"
 <rust-for-linux@...r.kernel.org>, "dri-devel@...ts.freedesktop.org"
 <dri-devel@...ts.freedesktop.org>, "Alexandre Courbot"
 <acourbot@...dia.com>, "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" <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>, "Timur
 Tabi" <ttabi@...dia.com>, "joel@...lfernandes.org"
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>,
 "Daniel Almeida" <daniel.almeida@...labora.com>,
 "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>, "Edwin
 Peer" <epeer@...dia.com>
Subject: Re: [PATCH v7.1 2/4] gpu: nova-core: bitfield: Move
 bitfield-specific code from register! into new macro

On Tue Oct 21, 2025 at 12:50 AM CEST, John Hubbard wrote:
> On 10/16/25 12:39 PM, John Hubbard wrote:
>> On 10/16/25 12:34 PM, Danilo Krummrich wrote:
>>> On Thu Oct 16, 2025 at 9:28 PM CEST, Joel Fernandes wrote:
>>>>> On Oct 16, 2025, at 1:48 PM, Yury Norov <yury.norov@...il.com> wrote:
>>>>> On Thu, Oct 16, 2025 at 11:13:21AM -0400, Joel Fernandes wrote:
>> ...
>>> While I'm not super opinionated for general bitfields, for the register!()
>>> infrastructure I very much prefer the hi:lo notation, as this is the common
>>> notation in datasheets and TRMs.
>>>
>>> However, if we use hi:lo, we should use it decending, i.e.:
> (restored from the email thread):
>
> 	bitfield! {
> 	    struct ControlReg {
> 	        7:5 state as u8 => State;
> 	        3:0 mode as u8 ?=> Mode;
> 	    }
> 	}>>
>> 
>> Sure, descending works.
>
> Oops! I need to correct myself. After reviewing most of Joel Fernandes'
> latest patchset ([PATCH 0/7] Pre-requisite patches for mm and irq in
> nova-core) [1], I remember that the HW documentation is written in
> ascending order.
>
> For one example (out of countless hundreds or thousands), please see [2].
> Considering that I actually pushed this file up to github just a few
> years ago, it's rather silly of me to forget this basic truth. :)
>
> We really want to stay close to the HW documentation, and so, all other
> things being (nearly) equal, this means that we should prefer ascending
> field order, if that's OK with everyone.

But that's OpenRM specific, I'm pretty sure when you look at internal datasheets
and TRMs you will find hi:lo with decending order, for instance [3] page 1672
(clicked a random location in the scroll bar. :).

Besides, I think that hi:lo with ascending order is confusing. It should either
be hi:lo decending or lo:hi ascending.

For registers the common one is the former.

> [1] https://lore.kernel.org/20251020185539.49986-1-joelagnelf@nvidia.com
> [2] https://github.com/NVIDIA/open-gpu-doc/blob/master/manuals/ampere/ga102/dev_ce.ref.txt
[3] https://developer.nvidia.com/downloads/orin-series-soc-technical-reference-manual/