i.MX ProcessorsのトピックRe: Question about vfpv3 and neon

Question about vfpv3 and neon

Wizard_Richard — Wed, 13 May 2015 08:46:24 GMT

Dear All,

I am confused about vfpv3 and neon.

As shown in i.MX 6Dual/6Quad Reference Manual, it supports neon and vfpv3, also in kernel defconfig, the related config is

"CONFIG_VFP=y

CONFIG_VFPv3=y

CONFIG_NEON=y".

Question1:

In L3.0.35_4.1.0_ER_SOURCE_BSP release package, the cflags is "-march=armv7-a -mfpu=vfpv3 -mfloat-abi=softfp",

but in android_jb4.3_1.1.0-ga_source release package, the arch_variant_cflags is "-march=armv7-a -mfpu=neon -mfloat-abi=softfp".

what is the difference?

Question2: Does the option only used in cflags? not uesd in ldflags?

For example:

arm-fsl-linux-gnueabi-gcc -c helloworld.c -march=armv7-a -mfpu=vfpv3 -mfloat-abi=softfp

arm-fsl-linux-gnueabi-gcc -o helloworld helloworld.o [-march=armv7-a -mfpu=vfpv3 -mfloat-abi=softfp]

Is it need to add the ldflags "-march=armv7-a -mfpu=vfpv3 -mfloat-abi=softfp" ?

Best Regards,

Richard

igorpadykov — Wed, 13 May 2015 10:53:44 GMT

Hi Richard

fpu is not implemented on i.MX6 platform, below part of i.MX6DQ RM

for flags one can look at attached gcc release note.

Best regards

igor

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Yuri — Thu, 14 May 2015 02:22:31 GMT

Basically, the ARM core of the i.MX6 supports both VFP and NEON :

NEON is a SIMD engine, the VFP is an Floating Point coprocessor.
"The Media Processing Engine (MPE) implements ARM NEON technology.

[...] The MPE also implements a VFPv3-D32 Floating-Point Unit."

We can apply NEON for single-precision floating-point operations for
several values in parallel. Under Linux one should use the same floating

point options for applications and kernel.

As for

"Is it need to add the ldflags "-march=armv7-a -mfpu=vfpv3 -mfloat-abi=softfp" ?"

- yes, since the mentioned options relates to C compiler.

Regards,

Yuri.