linux-kernel - Re: [PATCH v8 02/21] acpi: fix acpi_os

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Date:	Wed, 04 Feb 2015 16:38:59 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	Mark Langsdorf <mlangsdo@...hat.com>,
	linaro-acpi@...ts.linaro.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Yijing Wang <wangyijing@...wei.com>,
	Rob Herring <robh@...nel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Timur Tabi <timur@...eaurora.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Mark Salter <msalter@...hat.com>,
	Grant Likely <grant.likely@...aro.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	phoenix.liyi@...wei.com, Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <marc.zyngier@....com>,
	Jon Masters <jcm@...hat.com>, Mark Brown <broonie@...nel.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	Ashwin Chaugule <ashwinc@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v8 02/21] acpi: fix acpi_os_ioremap for arm64

On Wed, 2015-02-04 at 16:25 +0000, Russell King - ARM Linux wrote:
> I haven't experimented with it myself, but I think David Woodhouse
> has some experience in this area.

In many kernel configurations there are actually quite a lot of
functions that are never called, and I was quite surprised the first
time I played with this stuff.

There are a few ways of dealing with it. One is to use
-ffunction-section -fdata-sections --gc-sections as you noted. I once
also played with using GCC's --combine during the brief period that it
was supported and not *entirely* broken, with similar effects:
https://lwn.net/Articles/197097/

These days, the better answer is probably LTO. We could potentially
still look at --gc-sections, but I suspect we're better off using LTO
and just filing toolchain bugs until everything that --gc-sections
*would* have dropped is also dropped from the LTO build :)

Unless --gc-sections actually speeds up the build in a significant way;
a full LTO link of the kernel takes insane amounts of memory IIRC.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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