linux-kernel - Re: [PATCH v2 2/4] gpu: nova-core: gsp: clarify comments about invariants and pointer roles

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Message-Id: <DFZXYMSS9OOI.5NUPB8GCHRKD@nvidia.com>
Date: Wed, 28 Jan 2026 13:35:35 +0900
From: "Eliot Courtney" <ecourtney@...dia.com>
To: "Gary Guo" <gary@...yguo.net>, "Eliot Courtney" <ecourtney@...dia.com>,
 "Danilo Krummrich" <dakr@...nel.org>, "Alexandre Courbot"
 <acourbot@...dia.com>, "Alice Ryhl" <aliceryhl@...gle.com>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Alistair Popple"
 <apopple@...dia.com>
Cc: <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
 "dri-devel" <dri-devel-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/4] gpu: nova-core: gsp: clarify comments about
 invariants and pointer roles

On Tue Jan 27, 2026 at 3:04 AM JST, Gary Guo wrote:
>>          // - We will only access the driver-owned part of the shared memory.
>>          // - Per the safety statement of the function, no concurrent access will be performed.
>>          let gsp_mem = &mut unsafe { self.0.as_slice_mut(0, 1) }.unwrap()[0];
>> -        // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `<= MSGQ_NUM_PAGES`.
>> +        // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `< MSGQ_NUM_PAGES`.
>
> Can this just be `tx < MSGQ_NUM_PAGES`?

In previous discussion[1] it's been noted that it's important to
explain why the preconditions are satisfied, not just what they are,
so that's the reason I kept this in. Happy to hear arguments either
way though.

[1]: https://lore.kernel.org/all/CAH5fLgg0O=t2T2MQH2hvm4eZnCOa_NffzRw=XZPi9+7+=XsmRg@mail.gmail.com/

>>      // # Invariants
>>      //
>> -    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
>> +    // - The returned value is between `0` and `MSGQ_NUM_PAGES - 1`, inclusive.
>
> I wonder if this can be `is within 0..MSGQ_NUM_PAGES`. What do others think?

I think this is very reasonable, since this is part of the rust
range syntax so it should be understandable. I also considered the
mathematical syntax `[0, MSGQ_NUM_PAGES)`, but not sure if this would
be conventional - it does seem that this notation is used in a bunch
of places though. Will apply your suggestion in the next version unless
there is a definitive convention for this.