linux-kernel - Re: [PATCH v5 02/12] gpu: nova-core: move GSP boot code to a dedicated method

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Message-Id: <DCS59IDCIKH1.2M3I6H0NVD0RG@nvidia.com>
Date: Sun, 14 Sep 2025 10:49:40 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>, "John Hubbard"
 <jhubbard@...dia.com>
Cc: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>, "Danilo Krummrich"
 <dakr@...nel.org>, "Alexandre Courbot" <acourbot@...dia.com>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "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>, "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>, "Alistair
 Popple" <apopple@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v5 02/12] gpu: nova-core: move GSP boot code to a
 dedicated method

On Sun Sep 14, 2025 at 7:06 AM JST, Joel Fernandes wrote:
> On Sat, Sep 13, 2025 at 02:29:54PM -0700, John Hubbard wrote:
> [..]
>
>> > 
>> > I would suggest taking a look at our website and the links there (like
>> > issue #2) -- what we are doing upstream Rust is documented.
>> 
>> ...and my question was asked before reading through issue #2. So your
>> and Danilo's responses seem to be saying that there is already some
>> understanding that this is an area that could be improved.
>> 
>> Good!
>> 
>> I believe "issue #2" refers to this, right?
>> 
>>    https://github.com/Rust-for-Linux/linux/issues/2
>> 
>> That's going to take some time to figure out if it interects
>> what I was requesting, but I'll have a go at it.
>
> Indeed, kudos to rust-for-linux community for working on missing Rust
> features and on pinning itself.
>
>> > 
>> > (Danilo gave you a direct link, but I mention it this way because
>> > there are a lot of things going on, and it is worth a look and perhaps
>> > you may find something interesting you could help with).
>> > 
>> > > except to satisfy paranoia
>> > 
>> > Using unsafe code everywhere (or introducing unsoundness or UB for
>> > convenience) would defeat much of the Rust for Linux exercise.
>> > 
>> 
>> Yes. It's only "paranoia" if the code is bug-free. So Rust itself
>> naturally will look "a little" paranoid, that's core to its mission. :)
>
> This seems to be taken out-of-context, I said "paranoia" mainly because I am
> not sure if excessive use of pinning may tend to cause other problems. The
> "paranoia" is about over-usage of pinning. Personally, I don't prefer to pin
> stuff in my code until I absolutely need to, or when I start having needs for
> pinning, like using spinlocks. Maybe I am wrong, but the way I learnt Rust,
> data movement is baked into it. I am not yet confident pinning will not
> constraint Rust code gen -- but that could just be a part of my learning
> journey that I have to convince myself it is Ok to do so in advance of
> actually requiring it even if you simply hypothetically anticipate needing it
> (as Danilo pointed out that in practice this is not an issue and I do tend to
> agree with Miguel and Danilo because they are usually right :-D). I am
> researching counter examples :-)

You can look at the definition for `Pin` in [1], but it is so short we
can paste it here:

    #[repr(transparent)]
    #[derive(Copy, Clone)]
    pub struct Pin<Ptr> {
        pointer: Ptr,
    }

There isn't much getting in the way of optimized code generation - its
purpose is simply to constraint the acquisition of mutable references to
prevent moving the pointee out.

I started this patchset a little bit skeptical about the need to pin so
many things, but after seeing the recent additions to `pin_init` and
rewriting the code as Danilo suggested, it starteds to click. The
supposed restrictions are in practice avoided by embracing the concept
fully, and in the end I got that feeling (familiar when writing Rust) of
being guided towards the right design - a bit like playing bowling with
gutter guards.

Yes, that means redesigning and rebasing our code, but that's also the
cost of learning a new language.

And yes, things can still be a little bit rough around the edges, but
there is awareness and action taken to address these issues, at the
compiler level when relevant. This makes me confident for the future.

[1] https://doc.rust-lang.org/src/core/pin.rs.html#1094