Re: [PATCH v2 00/12] lib/crc: improve how arch-optimized code is integrated

[Julian Calaby]

Hi Eric,

On Sun, Jun 8, 2025 at 6:07 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> This series is also available at:
>
>     git fetch https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git lib-crc-arch-v2
>
> This series improves how lib/crc supports arch-optimized code.  First,
> instead of the arch-optimized CRC code being in arch/$(SRCARCH)/lib/, it
> will now be in lib/crc/$(SRCARCH)/.  Second, the API functions (e.g.
> crc32c()), arch-optimized functions (e.g. crc32c_arch()), and generic
> functions (e.g. crc32c_base()) will now be part of a single module for
> each CRC type, allowing better inlining and dead code elimination.  The
> second change is made possible by the first.
>
> As an example, consider CONFIG_CRC32=m on x86.  We'll now have just
> crc32.ko instead of both crc32-x86.ko and crc32.ko.  The two modules
> were already coupled together and always both got loaded together via
> direct symbol dependency, so the separation provided no benefit.
>
> Note: later I'd like to apply the same design to lib/crypto/ too, where
> often the API functions are out-of-line so this will work even better.
> In those cases, for each algorithm we currently have 3 modules all
> coupled together, e.g. libsha256.ko, libsha256-generic.ko, and
> sha256-x86.ko.  We should have just one, inline things properly, and
> rely on the compiler's dead code elimination to decide the inclusion of
> the generic code instead of manually setting it via kconfig.
>
> Having arch-specific code outside arch/ was somewhat controversial when
> Zinc proposed it back in 2018.  But I don't think the concerns are
> warranted.  It's better from a technical perspective, as it enables the
> improvements mentioned above.  This model is already successfully used
> in other places in the kernel such as lib/raid6/.  The community of each
> architecture still remains free to work on the code, even if it's not in
> arch/.  At the time there was also a desire to put the library code in
> the same files as the old-school crypto API, but that was a mistake; now
> that the library is separate, that's no longer a constraint either.

Quick question, and apologies if this has been covered elsewhere.

Why not just use choice blocks in Kconfig to choose the compiled-in
crc32 variant instead of this somewhat indirect scheme?

This would keep the dependencies grouped by arch and provide a single
place to choose whether the generic or arch-specific method is used.

It would also allow for alternatives if that ever becomes a thing and
compile testing of the arch-specific variants if that even offers any
actual value.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@xxxxxxxxx
Profile: http://www.google.com/profiles/julian.calaby/