On Tue, Jun 10, 2025 at 08:36:39AM +1000, Julian Calaby wrote: > Hi Eric, > > On Tue, Jun 10, 2025 at 5:49 AM Eric Biggers <ebigg...@kernel.org> wrote: > > > > On Mon, Jun 09, 2025 at 06:15:24PM +1000, Julian Calaby wrote: > > > Hi Eric, > > > > > > On Sun, Jun 8, 2025 at 6:07 AM Eric Biggers <ebigg...@kernel.org> 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's not clear exactly what you're suggesting, but it sounds like you're > > complaining about this: > > > > config CRC32_ARCH > > bool > > depends on CRC32 && CRC_OPTIMIZATIONS > > default y if ARM && KERNEL_MODE_NEON > > default y if ARM64 > > default y if LOONGARCH > > default y if MIPS && CPU_MIPSR6 > > default y if PPC64 && ALTIVEC > > default y if RISCV && RISCV_ISA_ZBC > > default y if S390 > > default y if SPARC64 > > default y if X86 > > I was suggesting something roughly like: > > choice > prompt "CRC32 Variant" > depends on CRC32 && CRC_OPTIMIZATIONS > > config CRC32_ARCH_ARM_NEON > bool "ARM NEON" > default y > depends ARM && KERNEL_MODE_NEON > > ... > > config CRC32_GENERIC > bool "Generic" > > endchoice > > > This patchset strikes a balance where the vast majority of the > > arch-specific CRC > > code is isolated in lib/crc/$(SRCARCH), and the exceptions are just > > lib/crc/Makefile and lib/crc/Kconfig. I think these exceptions make sense, > > given that we're building a single module per CRC variant. We'd have to go > > through some hoops to isolate the arch-specific Kconfig and Makefile > > snippets > > into per-arch files, which don't seem worth it here IMO. > > I was only really concerned with the Kconfig structure, I was > expecting Kbuild to look roughly like this: (filenames are wrong) > > crc32-y += crc32-base.o > crc32-$(CRC32_ARCH_ARM_NEON) += arch/arm/crc32-neon.o > ... > crc32-$(CRC32_GENERIC) += crc32-generic.o > > but yeah, your proposal here has grown on me now that I think about it > and the only real "benefit" mine has is that architectures can display > choices for variants that have Kconfig-visible requirements, which > probably isn't that many so it wouldn't be useful in practice. > > Thanks for answering my question,
The CRC32 implementation did used to be user-selectable, but that was already removed in v6.14 (except for the coarse-grained knob CONFIG_CRC_OPTIMIZATIONS that remains and can be disabled only when CONFIG_EXPERT=y) since the vast majority of users simply want the optimized CRC32 code enabled. The fact that it wasn't just enabled by default was a longstanding bug. - Eric
