linux-kernel - Re: [PATCH v3 2/6] scripts/gen-crc-consts: add gen-crc-consts.py

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Message-ID: <20250206200843.GA1237@sol.localdomain>
Date: Thu, 6 Feb 2025 12:08:43 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	x86@...nel.org, linux-block@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>, Keith Busch <kbusch@...nel.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	"Martin K . Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v3 2/6] scripts/gen-crc-consts: add gen-crc-consts.py

On Thu, Feb 06, 2025 at 07:31:17PM +0000, David Laight wrote:
> On Wed,  5 Feb 2025 23:39:44 -0800
> Eric Biggers <ebiggers@...nel.org> wrote:
> 
> > From: Eric Biggers <ebiggers@...gle.com>
> > 
> > Add a Python script that generates constants for computing the given CRC
> > variant(s) using x86's pclmulqdq or vpclmulqdq instructions.
> > 
> > This is specifically tuned for x86's crc-pclmul-template.S.  However,
> > other architectures with a 64x64 => 128-bit carryless multiplication
> > instruction should be able to use the generated constants too.  (Some
> > tweaks may be warranted based on the exact instructions available on
> > each arch, so the script may grow an arch argument in the future.)
> > 
> > The script also supports generating the tables needed for table-based
> > CRC computation.  Thus, it can also be used to reproduce the tables like
> > t10_dif_crc_table[] and crc16_table[] that are currently hardcoded in
> > the source with no generation script explicitly documented.
> > 
> > Python is used rather than C since it enables implementing the CRC math
> > in the simplest way possible, using arbitrary precision integers.  The
> > outputs of this script are intended to be checked into the repo, so
> > Python will continue to not be required to build the kernel, and the
> > script has been optimized for simplicity rather than performance.
> 
> It might be better to output #defines that just contain array
> initialisers rather than the definition of the actual array itself.
> 
> Then any code that wants the values can include the header and
> just use the constant data it wants to initialise its own array.
> 
> 	David

The pclmul constants use structs, not arrays.  Maybe you are asking for the
script to only generate the struct initializers?  This suggestion seems a bit
more complicated than just having everything in one place.  It would allow
putting the struct definitions in the CRC-variant-specific files while keeping
the struct initializers all in one file, so __maybe_unused would no longer need
to be used on the definitions.  But the actual result would be the same, just
achieved in what seems like a slightly more difficult way.

- Eric