linux-kernel - Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi

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Date:	Tue, 24 Nov 2015 20:35:16 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Peter Maydell <peter.maydell@...aro.org>,
	Måns Rullgård <mans@...sr.com>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Christopher Covington <cov@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi_{u}idiv library
 functions

On Tue, Nov 24, 2015 at 12:07:30PM -0800, Stephen Boyd wrote:
> Ok. Presumably the order of arch-$(CONFIG) lines in the Makefile
> are done in an order to allow the build to degrade to the lowest
> common denominator among architecture support.

Correct.  Make processes the directives in the order listed in the
makefile, which means that a variable final value results from its
last assignment.

> CPU_V7 selects
> CPU_32v7 and we're using that config to select -march=armv7-a in
> the Makefile. The patch currently uses CPU_32v7VE to select
> -march=armv7ve. If CPU_V7VE selects CPU_V7 we'll never be able to
> use -march=armv7ve because CPU_V7 will be selecting CPU_32v7 and
> that will come after CPU_32v7VE in the Makefile.

Right, but look at how the V6K stuff is handled:

arch-$(CONFIG_CPU_32v6)         =-D__LINUX_ARM_ARCH__=6 $(call cc-option,-march=armv6,-march=armv5t -Wa$(comma)-march=armv6)
# Only override the compiler option if ARMv6. The ARMv6K extensions are
# always available in ARMv7
ifeq ($(CONFIG_CPU_32v6),y)
arch-$(CONFIG_CPU_32v6K)        =-D__LINUX_ARM_ARCH__=6 $(call cc-option,-march=armv6k,-march=armv5t -Wa$(comma)-march=armv6k)
endif

We'd need to do something similar for v7VE as well.  As we're getting
more of this, I'd suggest we move to:

arch-v7a-y			=$(call cc-option,-march=armv7-a,-march=armv5t -Wa$(comma)-march=armv7-a)
arch-v7a-$(CONFIG_CPU_32v7VE)	=... whatever it was...
arch-$(CONFIG_CPU_32v7)		=-D__LINUX_ARM_ARCH__=7 $(arch-v7a-y)
arch-v6-y			=$(call cc-option,-march=armv6,-march=armv5t -Wa$(comma)-march=armv6)
arch-v6-$(CONFIG_CPU_32v6K)	=$(call cc-option,-march=armv6k,-march=armv5t -Wa$(comma)-march=armv6k)
arch-$(CONFIG_CPU_32v6)		=-D__LINUX_ARM_ARCH__=6 $(arch-v6-y)

> My understanding is that we want to support CPU_V7VE without
> CPU_V7 enabled so that it uses the idiv instructions in that
> configuration. When V7VE and V7 are both enabled, we should
> degrade to the aeabi functions, and the same is true for when
> V7VE is disabled.

Let me have another look at this, it's been a while since I touched these
options...

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