Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

Herbert Xu
2022-04-12T10:55:39Z

On Tue, Apr 12, 2022 at 10:32:58AM +0100, Catalin Marinas wrote:
>
> At a quick grep, most cra_alignmask values are currently 15 or smaller.
> I'm not convinced the driver needs to know about the CPU cache
> alignment. We could set cra_alignmask to CRYPTO_MINALIGN but that would
> incur unnecessary overhead via function like setkey_unaligned() when the
> arch_kmalloc_minalign() was already sufficient for DMA safety.
>
> Maybe I miss some use-cases or I focus too much on DMA safety.

The alignment was never designed for DMA. It's mainly for CPUs
that provide accelerated instructions that require a certain
amount of alignment, most commonly 16 bytes.

Therefore CRYPTO_MINALIGN was never meant to be a guarantee for
DMA alignment. Any drivers relying on this is simply broken.

I understand that on ARM for historical reasons you have had a
situation that CRYPTO_MINALIGN happened to be large enough to
guarantee DMA alignment. I totally agree with your patch to
try to fix this but it should not penalise other architectures
and raise the CRYPTO_MINALIGN unnecessarily.

I think if CRYPTO_MINALIGN makes those drivers work then so
should cra_alignmask. And that would be a relatively easy
patch to do.

Cheers,
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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