[RFC PATCH v2 1/2] cpuidle: Add common init interface and idle functionality

On Wed, Jan 4, 2012 at 3:15 PM, Rob Lee [off-list ref] wrote:

On 22 December 2011 16:57, Rob Herring [off-list ref] wrote:

quoted

On 12/14/2011 01:02 AM, Robert Lee wrote:

quoted

+static struct cpuidle_driver exynos4_idle_driver = {
+ ? ? .name ? ? ? ? ? = "exynos4_idle",
+ ? ? .owner ? ? ? ? ?= THIS_MODULE,
+ ? ? .states[0] = {
+ ? ? ? ? ? ? .enter ? ? ? ? ? ? ? ? ?= cpuidle_def_idle,
? ? ? ? ? ? ? .exit_latency ? ? ? ? ? = 1,
? ? ? ? ? ? ? .target_residency ? ? ? = 100000,
? ? ? ? ? ? ? .flags ? ? ? ? ? ? ? ? ?= CPUIDLE_FLAG_TIME_VALID,
? ? ? ? ? ? ? .name ? ? ? ? ? ? ? ? ? = "IDLE",
? ? ? ? ? ? ? .desc ? ? ? ? ? ? ? ? ? = "ARM clock gating(WFI)",
? ? ? },

As this is just plain wfi and shouldn't really be different per
platform, it would be nice to get rid of all of this state info. Perhaps
a macro with all the data since each driver needs to init each state.
The target residency value looks kind of suspect. You should only go to
wfi if you expect to be idle for 100ms?

Ok, I'll look at add a ARM specific macro for a generic WFI state in v3.

For the target_residency value, I don't understand why the values
being used are there other than some of the original ARM platform
implementations were based on the Intel implementations per their
comments, and the Intel value was used. ?From the comments in
drivers/cpuidle/governors/menu.c: "state entry and exit have an energy
cost, and a certain amount of time in the ?C state is required to
actually break even on this cost. CPUIDLE provides us this duration in
the target_residency field". ?The governor code uses it accordingly.
I'm not aware that a pure WFI only state uses additional energy to
enter and exit compared to spinning in a loop, so I think the
"target_residency" of a pure WFI state value should 0 or 1. ?If a
platform skips a pure WFI idle state and also implements additional
platform specific power savings in their "WFI" idle state, then they
may need to adjust exit_latency and target_residency accordingly.

Extra data point: on OMAP4 we chose .target_residency = 5 for simple WFI state:
http://git.omapzoom.org/?p=kernel/omap.git;a=blob;f=arch/arm/mach-omap2/cpuidle44xx.c;h=383944076085f808b8764baf11df0589229010e5;hb=p-android-omap-3.0#l113

Regards,
Mike