linux-kernel - Re: [PATCH v3 16/19] nova-core: Add support for VBIOS ucode extraction for boot

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Message-ID: <CAPM=9typcavVsj-w_4zaBkU=eo-hsOagHn4cMekCsXPHwLK3Aw@mail.gmail.com>
Date: Wed, 21 May 2025 07:32:17 +1000
From: Dave Airlie <airlied@...il.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: 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 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Simona Vetter <simona@...ll.ch>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Jonathan Corbet <corbet@....net>, John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>, 
	Timur Tabi <ttabi@...dia.com>, Alistair Popple <apopple@...dia.com>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, Shirish Baskaran <sbaskaran@...dia.com>
Subject: Re: [PATCH v3 16/19] nova-core: Add support for VBIOS ucode
 extraction for boot

On Wed, 21 May 2025 at 04:13, Joel Fernandes <joelagnelf@...dia.com> wrote:
>
>
>
> On 5/20/2025 11:36 AM, Danilo Krummrich wrote:
> >>> If you want a helper type with Options while parsing that's totally fine, but
> >>> the final result can clearly be without Options. For instance:
> >>>
> >>>     struct Data {
> >>>        image: KVec<u8>,
> >>>     }
> >>>
> >>>     impl Data {
> >>>        fn new() -> Result<Self> {
> >>>           let parser = DataParser::new();
> >>>
> >>>           Self { image: parser.parse()? }
> >>>        }
> >>>
> >>>        fn load_image(&self) {
> >>>           ...
> >>>        }
> >>>     }
> >>>
> >>>     struct DataParser {
> >>>        // Only some images have a checksum.
> >>>        checksum: Option<u64>,
> >>>        // Some images have an extra offset.
> >>>        offset: Option<u64>,
> >>>        // Some images need to be patched.
> >>>        patch: Option<KVec<u8>>,
> >>>        image: KVec<u8>,
> >>>     }
> >>>
> >>>     impl DataParser {
> >>>        fn new() -> Self {
> >>>           Self {
> >>>              checksum: None,
> >>>              offset: None,
> >>>              patch: None,
> >>>              bytes: KVec::new(),
> >>>           }
> >>>        }
> >>>
> >>>        fn parse(self) -> Result<KVec<u8>> {
> >>>           // Fetch all the required data.
> >>>           self.fetch_checksum()?;
> >>>           self.fetch_offset()?;
> >>>           self.fetch_patch()?;
> >>>           self.fetch_byes()?;
> >>>
> >>>           // Doesn't do anything if `checksum == None`.
> >>>           self.validate_checksum()?;
> >>>
> >>>           // Doesn't do anything if `offset == None`.
> >>>           self.apply_offset()?;
> >>>
> >>>           // Doesn't do anything if `patch == None`.
> >>>           self.apply_patch()?;
> >>>
> >>>           // Return the final image.
> >>>           self.image
> >>>        }
> >>>     }
> >>>
> >>> I think the pattern here is the same, but in this example you keep working with
> >>> the DataParser, instead of a new instance of Data.
> >> I think this would be a fundamental rewrite of the patch. I am Ok with looking
> >> into it as a future item, but right now I am not sure if it justifies not using
> >> Option for these few. There's a lot of immediate work we have to do for boot,
> >> lets please not block the patch on just this if that's Ok with you. If you want,
> >> I could add a TODO here.
> >
> > Honestly, I don't think it'd be too bad to fix this up. It's "just" a bit of
> > juggling fields and moving code around. The actual code should not change much.
> >
> > Having Option<T> where the corresponding value T isn't actually optional is
> > extremely confusing and makes it hard for everyone, but especially new
> > contributors, to understand the code and can easily trick people into taking
> > wrong assumptions.
> >
> > Making the code reasonably accessible for (new) contributors is one of the
> > objectives of nova and one of the learnings from nouveau.

I just want to back Danilo up on this concept as well.

When I did the experiments code, I faced the not fully constructed
object problem a lot, and I tried to resist the C pattern of Option<>
all the things, it's a very C based thing where we create an object
then initialise it as we go, and it's not a great pattern to have for
rust code.

I'm not a huge fan of constructor/builder objects either if they can
be avoided, please do, and I tried to also avoid proliferating them,
but I think for most things we can build the pieces and then the final
object as we go, it just requires doing so from the start, and not
giving into the Option<> pattern.

Dave.