linux-kernel - Re: [PATCH v3 02/11] gpu: nova-core: move GSP boot code out of `Gpu` constructor

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Message-Id: <DCIZ5VVLACXO.1L0QTYM5YVRQV@nvidia.com>
Date: Wed, 03 Sep 2025 16:08:50 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: "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>, "John Hubbard"
 <jhubbard@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Joel
 Fernandes" <joelagnelf@...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 v3 02/11] gpu: nova-core: move GSP boot code out of
 `Gpu` constructor

On Wed Sep 3, 2025 at 4:53 AM JST, Danilo Krummrich wrote:
> On Tue Sep 2, 2025 at 4:31 PM CEST, Alexandre Courbot wrote:
>> diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
>> index 274989ea1fb4a5e3e6678a08920ddc76d2809ab2..1062014c0a488e959379f009c2e8029ffaa1e2f8 100644
>> --- a/drivers/gpu/nova-core/driver.rs
>> +++ b/drivers/gpu/nova-core/driver.rs
>> @@ -6,6 +6,8 @@
>>  
>>  #[pin_data]
>>  pub(crate) struct NovaCore {
>> +    // Placeholder for the real `Gsp` object once it is built.
>> +    pub(crate) gsp: (),
>>      #[pin]
>>      pub(crate) gpu: Gpu,
>>      _reg: auxiliary::Registration,
>> @@ -40,8 +42,14 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
>>          )?;
>>  
>>          let this = KBox::pin_init(
>> -            try_pin_init!(Self {
>> +            try_pin_init!(&this in Self {
>>                  gpu <- Gpu::new(pdev, bar)?,
>> +                gsp <- {
>> +                    // SAFETY: `this.gpu` is initialized to a valid value.
>> +                    let gpu = unsafe { &(*this.as_ptr()).gpu };
>> +
>> +                    gpu.start_gsp(pdev)?
>> +                },
>
> Please use pin_chain() [1] for this.

Sorry, but I couldn't figure out how I can use pin_chain here (and
couldn't find any relevant example in the kernel code either). Can you
elaborate a bit?

>
> More in general, unsafe code should be the absolute last resort. If we add new
> unsafe code I'd love to see a comment justifying why there's no other way than
> using unsafe code for this, as we agreed in [2].
>
> I did a quick grep on this series and I see 21 occurrences of "unsafe", if I
> substract the ones for annotations and for FromBytes impls, it's still 9 new
> ones. :(
>
> Do we really need all of them?

I've counted 16 uses of `unsafe`. :)

- 3 in the bindgen-generated code (these can't be avoided),
- 7 to implement `FromBytes`,
- 1 to work around the fact that `FromBytes` doesn't work on slices yet
  (maybe that one can be removed)
- 5 as a result of intra-dependencies in PinInit initializers (which we
  might be able to remove if I figure out how to use `pin_chain`).

So best-case scenario would be that we will be down to 10 that are truly
unavoidable.