linux-kernel - Re: [PATCH v2 2/3] rust: dma: convert the read/write macros to return Result

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Message-ID: <D93BX5NEOSC8.3NOVI9GMDJEXD@proton.me>
Date: Thu, 10 Apr 2025 22:58:10 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, a.hindborg@...nel.org, rust-for-linux@...r.kernel.org, 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>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Valentin Obst <kernel@...entinobst.de>, open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, airlied@...hat.com, "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2 2/3] rust: dma: convert the read/write macros to return Result

On Thu Apr 10, 2025 at 5:34 PM CEST, Danilo Krummrich wrote:
> On Thu, Apr 10, 2025 at 03:11:01PM +0000, Benno Lossin wrote:
>> On Thu Apr 10, 2025 at 1:54 PM CEST, Danilo Krummrich wrote:
>> > On Thu, Apr 10, 2025 at 11:58:17AM +0300, Abdiel Janulgue wrote:
>> >> @@ -78,13 +74,14 @@ impl Drop for DmaSampleDriver {
>> >>      fn drop(&mut self) {
>> >>          dev_info!(self.pdev.as_ref(), "Unload DMA test driver.\n");
>> >>  
>> >> -        let _ = || -> Result {
>> >> -            for (i, value) in TEST_VALUES.into_iter().enumerate() {
>> >> -                assert_eq!(kernel::dma_read!(self.ca[i].h), value.0);
>> >> -                assert_eq!(kernel::dma_read!(self.ca[i].b), value.1);
>> >> -            }
>> >> -            Ok(())
>> >> -        }();
>> >> +        for (i, value) in TEST_VALUES.into_iter().enumerate() {
>> >> +            let val0 = kernel::dma_read!(self.ca[i].h);
>> >> +            let val1 = kernel::dma_read!(self.ca[i].b);
>> >> +            assert!(val0.is_ok());
>> >> +            assert!(val1.is_ok());
>> >> +            assert_eq!(val0.unwrap(), value.0);
>> >> +            assert_eq!(val1.unwrap(), value.1);
>> >
>> > Maybe use if-let to avoid the unwrap?
>> >
>> > 	if let Ok(val0) = val0 {
>> > 	   assert_eq!(val0, value.0);
>> > 	}
>> >
>> > I know it's a bit pointless, since we know it must be ok, but the educational
>> > message of the example should be to check and not to unwrap, so maybe that's
>> > better.
>> 
>> The if-let will silently ignore any errors, so I don't think that it's
>> fit for example code either.
>
> Yes, but we still have the assert!() before, so the full sequence would be:
>
> 	assert!(val0.is_ok());
>
> 	if let Ok(val0) = val0 {
> 	   assert_eq!(val0, value.0);
> 	}

Ah right, missed that.

> The intention would be to avoid patterns that shouldn't be used in "real" code;
> assert!() should be obvious not to use for real code.

Yeah, I'm not sure if this is that valuable. I think having "real code"
is better, but I don't have any idea what to do in this case.

Why does this sample do the validation in the `drop` method in the first
place? I guess the same code on the C side would do this in `remove` or
whatever the equivalent thing is there, but would there be the option to
report an error? Or is `remove` an infallible operation? In that case
`assert!` probably is still the best option.

---
Cheers,
Benno