linux-kernel - Re: gpu: nova-core: arm32 build errors

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Message-Id: <DCEBQZ7WA1KI.1G8TNWH32QWG@kernel.org>
Date: Thu, 28 Aug 2025 21:57:16 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alexandre Courbot"
 <acourbot@...dia.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: gpu: nova-core: arm32 build errors

On Thu Aug 28, 2025 at 9:31 PM CEST, Miguel Ojeda wrote:
> On Thu, Aug 28, 2025 at 9:24 PM Danilo Krummrich <dakr@...nel.org> wrote:
>>
>> Maybe I spoke too soon, it's actually pretty painful to keep 32-bit
>> compatibility, even though it would be nice for testing purposes.
>>
>> I'll paste the diff to fix it below, I think that makes it obvious why I say
>> that.
>>
>> Instead, we should really just depend on CONFIG_64BIT (which implies
>> ARCH_DMA_ADDR_T_64BIT).
>
> Yeah, it isn't great.
>
> If it were just that, maybe it it is worth it (and a `DmaAddress`
> newtype, not just a typedef, could perhaps be nice anyway?)

What do you have in mind what the newtype can do?

I assume the idea is to make it provide methods {to,from}_u64, where to_u64()
has to be fallible? This would be an improvement, but not really solve the issue
entirely.

The annoying part really is

	pub(super) fn read_sysmem_flush_page_ga100(bar: &Bar0) -> DmaAddress {
	    let addr = u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR::read(bar).adr_39_08())
	        << FLUSH_SYSMEM_ADDR_SHIFT
	        | u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR_HI::read(bar).adr_63_40())
	            << FLUSH_SYSMEM_ADDR_SHIFT_HI;

	    addr.try_into().unwrap_or_else(|_| {
	        kernel::warn_on!(true);

	        0
	    })
	}

i.e. when we read a u64 from registers, but we know that what we've orignally
written there is a DmaAddress, so we can assume that a cast to DmaAddress is
fine.

But this really depends on driver specific semantics.

> but if
> you think it will become increasingly painful later, then it may be
> best to focus on what matters.

We'll have a couple more such cases for sure; I think being able to assume that
DmaAddress is always 64-bit will result in simpler and less distracting code.

But if we can come up with a good idea to deal with this with a DmaAddress type;
I'm open to that.

> It is unlikely there is going to be actual users on a 32-bit platform, right?

Yeah, I doubt that someone could think it's a great idea to run a Turing+ GPU on
some 32-bit machine.