linux-kernel - Re: [PATCH v2] crypto: stm32 - remove crc32 and crc32c support

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Message-Id: <0c760b4f-4f52-4384-b663-3b08e4f92448@app.fastmail.com>
Date: Wed, 04 Jun 2025 10:38:20 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Eric Biggers" <ebiggers@...nel.org>, linux-kernel@...r.kernel.org
Cc: linux-crypto@...r.kernel.org, "Ard Biesheuvel" <ardb@...nel.org>,
 "Alexandre Torgue" <alexandre.torgue@...s.st.com>,
 "Fabien Dessenne" <fabien.dessenne@...s.st.com>,
 "Lionel DEBIEVE" <lionel.debieve@...s.st.com>,
 "Maxime Coquelin" <mcoquelin.stm32@...il.com>,
 linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH v2] crypto: stm32 - remove crc32 and crc32c support

On Sun, Jun 1, 2025, at 21:34, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
>
> Remove the crc32 and crc32c support from the stm32 driver.  Since it's
> not wired up to the CRC library, almost no CRC user in the kernel can
> actually be taking advantage of it, so it's effectively dead code.
>
> Support for this hardware could be migrated to the CRC library, but
> there doesn't seem to be much point.  This CRC engine is present only on
> a couple older SoCs that lacked CRC instructions.
>
> Even on those SoCs, it's probably not worthwhile.  This driver has to
> deal with things like locking and runtime power management that do not
> exist in software CRC code and are a source of bugs (as is clear from
> the commit log) and add significant overhead to the processing of short
> messages, which are common.  The patch that originally added this driver
> seemed to justify it based purely on a microbenchmark on Cortex-M7 on
> long messages, not a real use case.  These days, if this driver were to
> be used at all it would likely be on Cortex-A7 instead.  This CRC engine
> is also not supported by QEMU, making the driver not easily testable.
>
> Acked-by: Ard Biesheuvel <ardb@...nel.org>

Acked-by: Arnd Bergmann <arnd@...db.de>

As far as I can tell, the CRC unit was present on the original
24 MHz STM32F1 (Cortex-M3), where it probably had some merit, and
later chips including the STM32MP1 (Cortex-A7) just incorporated
the block for compatibility.

      Arnd