Re: [PATCH] crypto: arm64/crc32 - detect crc32 support in assembler

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 1 February 2017 at 09:07, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> On 27 January 2017 at 10:52, Will Deacon <will.deacon@arm.com> wrote:
>> On Fri, Jan 27, 2017 at 10:43:16AM +0000, Ard Biesheuvel wrote:
>>> On 27 January 2017 at 10:40, Matthias Brugger <mbrugger@suse.com> wrote:
>>> > Older compilers may not be able to detect the crc32 extended cpu type.
>>>
>>> What do you mean 'detect'? Could you describe the failure in more detail
>>> please?
>>>
>>> > Anyway only inline assembler code is used, which gets passed to the
>>> > assembler. This patch moves the crc detection to the assembler.
>>> >
>>> > Suggested-by: Alexander Graf <agraf@suse.de>
>>> > Signed-off-by: Matthias Brugger <mbrugger@suse.com>
>>> > ---
>>> >  arch/arm64/crypto/Makefile      | 2 --
>>> >  arch/arm64/crypto/crc32-arm64.c | 3 +++
>>> >  2 files changed, 3 insertions(+), 2 deletions(-)
>>> >
>>> > diff --git a/arch/arm64/crypto/Makefile b/arch/arm64/crypto/Makefile
>>> > index aa8888d7b744..0d779dac75cd 100644
>>> > --- a/arch/arm64/crypto/Makefile
>>> > +++ b/arch/arm64/crypto/Makefile
>>> > @@ -48,8 +48,6 @@ CFLAGS_aes-glue-ce.o  := -DUSE_V8_CRYPTO_EXTENSIONS
>>> >
>>> >  obj-$(CONFIG_CRYPTO_CRC32_ARM64) += crc32-arm64.o
>>> >
>>> > -CFLAGS_crc32-arm64.o   := -mcpu=generic+crc
>>> > -
>>> >  $(obj)/aes-glue-%.o: $(src)/aes-glue.c FORCE
>>> >         $(call if_changed_rule,cc_o_c)
>>> >
>>> > diff --git a/arch/arm64/crypto/crc32-arm64.c b/arch/arm64/crypto/crc32-arm64.c
>>> > index 6a37c3c6b11d..10f5dd075323 100644
>>> > --- a/arch/arm64/crypto/crc32-arm64.c
>>> > +++ b/arch/arm64/crypto/crc32-arm64.c
>>> > @@ -29,6 +29,9 @@ MODULE_AUTHOR("Yazen Ghannam <yazen.ghannam@linaro.org>");
>>> >  MODULE_DESCRIPTION("CRC32 and CRC32C using optional ARMv8 instructions");
>>> >  MODULE_LICENSE("GPL v2");
>>> >
>>> > +/* Request crc extension capabilities from the assembler */
>>> > +asm(".arch_extension crc");
>>> > +
>>>
>>> Will should confirm, but I think this is a recent feature in GAS for
>>> AArch64, so this may break older toolchains as well.
>>
>> Yes, the .arch_extension directive isn't universally supported by AArch64
>> gas so we can't rely on it unconditionally. The best bet is to check for
>> the support and, if it's not present, then disable whatever feature relies
>> on it. See the lseinstr variable in Makefile.
>>
>
> Actually, this driver has become somewhat redundant now that we have
> an alternative that combines an implementation based on 64x64
> polynomial multiplication with an implementation based on the CRC32
> instructions.
>
> I will propose a patch that makes the latter usable when only the
> CRC32 instructions are supported.

... although you still haven't explained what the actual problem is
that you are trying to solve.