Hello Russell,
First of all, thanks for the insight you've been providing. It helps a lot.
(And thanks to everyone who's been answering some of my questions.)
Russell King - ARM Linux wrote:
On Wed, Jan 28, 2015 at 02:16:21PM +0100, Mason wrote:
[snip udelay discussion]
Q2. Cortex A9 global and private timers
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0407f/CIHGECHJ.html
(What are private timers used for?)
The per-cpu private timers are mostly scheduling of threads.
Hmmm, I'm confused (again).
CONFIG_CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK means "use the global timer
for the scheduler clock", right? In that case, are the local timers
unused by Linux?
#ifdef CONFIG_CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK
sched_clock_register(gt_sched_clock_read, 64, gt_clk_rate);
#endif
Is there generic code to set up local timers? (If so, where?)
sched_clock_register only seems to appear in machine-specific code.
What are the pros/cons of global timer vs local timers?
Or is such a question irrelevant?
(Because they are used for different purposes?)
In my platform-specific code, there is a config option to choose between
1) the ARM global timer
2) a platform-specific timer (timer0)
Most platforms implement their own timer, because its really sexy for
hardware engineers to create yet another different timer implementation
which is soo much better than every other timer implementation that has
already been created. You wouldn't believe how many different ways that
there are to create a timer - and we still have people coming up with
new novel implementations!
There is trouble in paradise. I was planning to give the global timer a try,
instead of the platform-specific timer, until I noticed: "The global timer
is clocked by PERIPHCLK." And it turns PERIPHCLK is connected to the SoC's
CPU_CLK (the clock that drives the CPU cores); the same clock that is managed
by cpufreq. I imagine it's double plus ungood for precise time-keeping to
have frequency changes of the clocksource input?
Q3. Using the generic global timer implementation
So, how do I use that implementation?
(Is someone other than STMicro using it?)
To answer that question:
$ grep -rn ARM_GLOBAL_TIMER | grep -v SCHED_CLOCK
arch/arm/mach-vexpress/Kconfig:7: select ARM_GLOBAL_TIMER
arch/arm/mach-bcm/Kconfig:87: select ARM_GLOBAL_TIMER
arch/arm/mach-bcm/Kconfig:108: select ARM_GLOBAL_TIMER
arch/arm/mach-rockchip/Kconfig:14: select ARM_GLOBAL_TIMER
arch/arm/mach-sti/Kconfig:4: select ARM_GLOBAL_TIMER
arch/arm/mach-zynq/Kconfig:6: select ARM_GLOBAL_TIMER if !CPU_FREQ
There are indeed a few users; also I wonder why mach-zynq enables
ARM_GLOBAL_TIMER only if !CPU_FREQ. (Could it be that they have
the CPU CLOCK driving PERIPHCLK?)
Regards.
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