ARM: sched_clock: allow sched_clock to be selected at runtime

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.

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 <[email protected]>

Cc Krzysztof Halasa <[email protected]>

Cc Tony Lindgren <[email protected]>

Cc Eric Miao <[email protected]>

Cc Colin Cross <[email protected]>

Cc Erik Gilling <[email protected]>

Cc Olof Johansson <[email protected]>

Cc Linus Walleij <[email protected]>

Cc Sascha Hauer <[email protected]>

Cc Alessandro Rubini <[email protected]>

Cc STEricsson <[email protected]>

Cc Lennert Buytenhek <[email protected]>

Cc Nicolas Pitre <[email protected]>

Cc Ben Dooks <[email protected]>

Cc Kukjin Kim <[email protected]>

Cc Jamie Iles <[email protected]>

Quoted 19 lines · 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.

Quoted 16 lines

Cc Imre Kaloz <[email protected]>

Cc Krzysztof Halasa <[email protected]>

Cc Tony Lindgren <[email protected]>

Cc Eric Miao <[email protected]>

Cc Colin Cross <[email protected]>

Cc Erik Gilling <[email protected]>

Cc Olof Johansson <[email protected]>

Cc Linus Walleij <[email protected]>

Cc Sascha Hauer <[email protected]>

Cc Alessandro Rubini <[email protected]>

Cc STEricsson <[email protected]>

Cc Lennert Buytenhek <[email protected]>

Cc Nicolas Pitre <[email protected]>

Cc Ben Dooks <[email protected]>

Cc Kukjin Kim <[email protected]>

Cc Jamie Iles <[email protected]>

Tested-by Jamie Iles <[email protected]>

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.

Quoted 19 lines

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.

[...]

Review

Quoted 10 lines

(quoted diff) +10 -0

+unsigned long long notrace sched_clock(void)
+{
+	if (read_sched_clock) {
+		u32 cyc = read_sched_clock();
+		return cyc_to_sched_clock(&cd, cyc, sched_clock_mask);
+	}
+
+	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
+					* (NSEC_PER_SEC / HZ);
+}

This bothers me that this function has to include a conditional which will _always_ be evaluated as true except during early boot. Why not simply initializing read_sched_clock with a jiffy_shed_clock function instead, and let it be overriden during boot? This way the actual sched_clock() code will be more straight forward.

Of course there is the race where read_sched_clock might be updated before or after the corresponding clock data/mask are updated leading to funky results if you are lucky enough to call sched_clock() during that exact wrong moment. But either we don't care (switching from a jiffy to a hardware clock base is going to cause quite a glitch already anyway), or simply disabling IRQs during initialization should prevent the race (presuming no other CPUs are running at that point).

Review

Quoted 10 lines · Marc Zyngier

(quoted diff) +10 -0

+unsigned long long notrace sched_clock(void)
+{
+	if (read_sched_clock) {
+		u32 cyc = read_sched_clock();
+		return cyc_to_sched_clock(&cd, cyc, sched_clock_mask);
+	}
+
+	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
+					* (NSEC_PER_SEC / HZ);
+}

This bothers me that this function has to include a conditional which will _always_ be evaluated as true except during early boot. Why not simply initializing read_sched_clock with a jiffy_shed_clock function instead, and let it be overriden during boot? This way the actual sched_clock() code will be more straight forward.