linux-kernel - Re: [PATCH 17/31] gpu: nova-core: Hopper/Blackwell: calculate reserved FB heap size

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Message-ID: <64d20d0f-9d1d-4a33-8cad-4a3370fe7175@nvidia.com>
Date: Fri, 23 Jan 2026 15:56:26 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Gary Guo <gary@...yguo.net>, Timur Tabi <ttabi@...dia.com>,
 "dakr@...nel.org" <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...dia.com>,
 "lossin@...nel.org" <lossin@...nel.org>,
 "a.hindborg@...nel.org" <a.hindborg@...nel.org>,
 "boqun.feng@...il.com" <boqun.feng@...il.com>,
 "aliceryhl@...gle.com" <aliceryhl@...gle.com>, Zhi Wang <zhiw@...dia.com>,
 "simona@...ll.ch" <simona@...ll.ch>,
 "alex.gaynor@...il.com" <alex.gaynor@...il.com>,
 "ojeda@...nel.org" <ojeda@...nel.org>, "tmgross@...ch.edu"
 <tmgross@...ch.edu>,
 "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
 "bjorn3_gh@...tonmail.com" <bjorn3_gh@...tonmail.com>,
 Edwin Peer <epeer@...dia.com>, "airlied@...il.com" <airlied@...il.com>,
 Joel Fernandes <joelagnelf@...dia.com>,
 "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
 Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 17/31] gpu: nova-core: Hopper/Blackwell: calculate
 reserved FB heap size

On 1/21/26 8:10 AM, Gary Guo wrote:
> On Thu Dec 4, 2025 at 7:34 AM GMT, John Hubbard wrote:
>> On 12/3/25 12:48 PM, Timur Tabi wrote:
>>> On Tue, 2025-12-02 at 21:59 -0800, John Hubbard wrote:
>>>> +#[inline(always)]
>>>> +pub(crate) const fn const_align_up<const ALIGN: usize>(value: usize) -> usize {
>>>> +    build_assert!(ALIGN.is_power_of_two());
>>>> +    (value + (ALIGN - 1)) & !(ALIGN - 1)
>>>> +}
>>>
>>> So this is just like the issue I'm having with .next_multiple_of() in my patch #10.
>>>
>>> Shouldn't you have a check to make sure that value + (ALIGN - 1) doesn't overflow?  Since I need to
>>> align up to the nearest const power of two, I could use this function instead of align_up() and
>>> avoid testing for an error condition.
>>
>> Const items are special: they are evaluated at compile time, rather
>> than at runtime. And so this will fail to compile, for values that
>> would overflow.
>>
>> For example:
...

> Note that this is only true if the function is only ever called from compile
> time. If the const function is indeeed used during runtime, you'll lose these
> checks (or, depending on Kconfig, get turned into BUG).
> 
OK, well so far, it's only ever used at const/compile time, so it
is correct as it stands. However, if it ends up getting used at
runtime, it can be enhanced in that patch(set). So for now, I'll
just leave it alone.

thanks,
-- 
John Hubbard