[PATCH v3] ARM: sched_clock: allow sched_clock to be selected at runtime

[Marc Zyngier]

sched_clock() is yet another blocker on the road to the single
image. This patch implements an idea by Russell King:

http://www.spinics.net/lists/linux-omap/msg49561.html

Instead of asking the platform to implement both sched_clock()
itself and the rollover callback, simply register a read()
function, and let the ARM code care about sched_clock() itself,
the conversion to ns and the rollover. sched_clock() uses
this read() function as an indirection to the platform code.

This allow some simplifications and possibly some footprint gain
when multiple platforms are compiled in. Among the drawbacks,
the removal of the *_fixed_sched_clock optimization which could
negatively impact some platforms (sa1100, tegra, versatile
and omap).

Tested on 11MPCore, OMAP4 and Tegra.

Cc: Imre Kaloz <kaloz at openwrt.org>
Cc: Krzysztof Halasa <khc at pm.waw.pl>
Cc: Tony Lindgren <tony at atomide.com>
Cc: Eric Miao <eric.y.miao at gmail.com>
Cc: Colin Cross <ccross at android.com>
Cc: Erik Gilling <konkers at android.com>
Cc: Olof Johansson <olof at lixom.net>
Cc: Linus Walleij <linus.walleij at stericsson.com>
Cc: Sascha Hauer <kernel at pengutronix.de>
Cc: Alessandro Rubini <rubini at unipv.it>
Cc: STEricsson <STEricsson_nomadik_linux at list.st.com>
Cc: Lennert Buytenhek <kernel at wantstofly.org>
Cc: Nicolas Pitre <nico at fluxnic.net>
Cc: Ben Dooks <ben-linux at fluff.org>
Cc: Kukjin Kim <kgene.kim at samsung.com>
Tested-by: Jamie Iles <jamie at jamieiles.com>
Acked-by: Linus Walleij <linus.walleij at linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
Based on 3.1-rc4.