CONFIG_CRYPTO_SERPENT is not enabled

Describe the bug

The serpent encryption algorithm is not supported in the kernel.

Steps to reproduce the behaviour

1. cryptsetup benchmark
2. Results for serpent are N/A

Device (s)

Raspberry Pi 4 Mod. B

System

$ cat /etc/rpi-issue
Raspberry Pi reference 2023-05-03
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 7c750947a959fb626a70c09fd17c65815df192ac, stage2

$ vcgencmd version
Mar 17 2023 10:50:39 
Copyright (c) 2012 Broadcom
version 82f3750a65fadae9a38077e3c2e217ad158c8d54 (clean) (release) (start)

$ uname -a
Linux mainserver 6.1.74-v8+ #1 SMP PREEMPT Sun Jan 28 01:30:24 UTC 2024 aarch64 GNU/Linux

Logs

No response

Additional context

I am not entirely sure why the algorthm is disabled. I can reasonably come up with 2 options:

1. Its too slow due to not being properly supported on the hardware side
2. It doesnt work at all on some devices

To test both, I have created https://github.com/Aegeontis/rpi-linux-serpent. The repo compiles the kernel in exactly the same way as the upstream one, except enabling serpent support. I installed the compiled kernel from that repo (arm64 on rpi4) and everything works fine. Speeds are acceptable.

Search the kernel configuration for CONFIG_CRYPTO_SERPENT and compare the upstream Raspberry Pi build with the configuration used by the linked rpi-linux-serpent repository. Reproduce the issue with cryptsetup benchmark on a Raspberry Pi 4, then verify that serpent is no longer reported as N/A when the configuration is enabled.