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

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Message-ID: <Z_flA_DP5_1mG6h_@pollux>
Date: Thu, 10 Apr 2025 17:34:27 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
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 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);
	}

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.