linux-kernel - Re: Question about ARM FASTFPE

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Date:   Wed, 10 Jul 2019 18:54:06 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question about ARM FASTFPE

On Wed, Jul 10, 2019 at 5:23 PM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Wed, Jul 10, 2019 at 01:30:24PM +0900, Masahiro Yamada wrote:
> > Hi.
> >
> > I have a question about the following code
> > in arch/arm/Makefile:
> >
> >
> > # Do we have FASTFPE?
> > FASTFPE :=arch/arm/fastfpe
> > ifeq ($(FASTFPE),$(wildcard $(FASTFPE)))
> > FASTFPE_OBJ :=$(FASTFPE)/
> > endif
> >
> >
> > Since arch/arm/fastfpe does not exist in the upstream tree,
> > I guess this is a hook to compile downstream source code.
> >
> > If a user puts arch/arm/fastfpe/ into their local source tree,
> > Kbuild is supposed to compile the files in it.
> >
> > Is this correct?
> >
> >
> > If so, I am afraid this would not work for O= building.
> >
> > $(wildcard ...) checks if this directory exists in the *objtree*,
> > while scripts/Makefile.build needs to include
> > arch/arm/fastfpe/Makefile from *srctree*.
> >
> > I think the correct code should be like follows:
> >
> > # Do we have FASTFPE?
> > FASTFPE :=arch/arm/fastfpe
> > ifneq ($(wildcard $(srctree)/$(FASTFPE)),)
> > FASTFPE_OBJ :=$(FASTFPE)/
> > endif
> >
> >
> > Having said that, I am not sure this code is worth fixing.
> >
> > This code was added around v2.5.1.9,
>
> ... as a _result_ of a discussion and deciding not to upstream it,
> but to still allow its use.  Fastfpe is faster than nwfpe (so has
> a definite advantage for FP intensive applications) but we decided
> we didn't want two FP emulation codes in the kernel.  However, if
> someone wants to use it, it has to be built into the kernel, it
> can't be modular.


IMHO, the entry in Makefile and Kconfig should be removed
from upstream, then moved to a part of the fastfpe local patch.



-- 
Best Regards
Masahiro Yamada