linux-kernel - Re: Question about ARM FASTFPE

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Message-ID: <CAK7LNAT3NDem7_oTyPuDVC7QGY=HuyG-GBE9QQhUuuL_Q=CeNg@mail.gmail.com>
Date:   Wed, 10 Jul 2019 19:43:19 +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 7:02 PM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Wed, Jul 10, 2019 at 06:54:06PM +0900, Masahiro Yamada wrote:
> > 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.
>
> Nope.  It means that rather than it being merely a drop-in, it has
> to be maintained against changes to both these files.  Sorry, that's
> more work.


This is the motivation of upstreaming for everybody.

We never know the code that does not exist in upstream.
Downstream code must pay maintenance cost for ever.


-- 
Best Regards
Masahiro Yamada