On Fri, Mar 23, 2012 at 13:50:25, Ming Lei wrote: > On Wed, Mar 21, 2012 at 7:29 PM, Hiremath, Vaibhav <hvaibhav@xxxxxx> wrote: > > On Mon, Mar 19, 2012 at 17:14:30, Ming Lei wrote: > >> On Mon, Mar 19, 2012 at 7:11 PM, Hiremath, Vaibhav <hvaibhav@xxxxxx> wrote: > >> > > >> > I think you made very good point here. With the above patch, we are almost missing the capability of registering dmtimer as a clocksource for OMAP. > >> > It will always use 32k-counter, and never fall back to dmtimer. > >> > > >> > Then the only options we have here is, > >> > > >> > 1) Register both the timers, 32k-counter and dmtimer for clocksource; let > >> > Kernel pick up best rating clocksource out of these two. > >> > > >> > In case of OMAP1/2/3/4, kernel will use dmtimer, since it has better > >> > Rating. User can choose the 32k-counter clocksource via bootargs. > >> > > >> > Impact: without bootargs for clocksource selection, kernel will choose > >> > dmtimer, impacting loss of time during suspend/resume. > >> > > >> > > >> > 2) Let the current code be as is, means, the clocksource registration will > >> > Happened based on "#ifdef CONFIG_OMAP_32K_TIMER" and this option > >> > selection will be Controlled by Kconfig rules. > >> > >> How about the 3rd option? > >> > >> 3), take the way in your patch 1) at default, but will switch to > >> register dmtimer > >> directly and bypass 32k-counter if user need it via kernel parameter. > >> > >> As far as I can think of, the situations required for dmtimer are high-frequency > >> perf sample and high precision trace points, so looks it is OK to take > >> 32k-counter > >> at default. > >> > > But if you register both the timers (dmtimer & 32ksync), then initially kernel will only pick up dmtimer, as this has better rating. And late in > > Looks not so, I found that 32ksync is always selected as the default > clocksource if both are registered. > No. Kernel always chooses better-rated timer/counter for clocksource. Please refer to the file "kernel/time/clocksource.c" When you register any clocksource, the sequence is ... clocksource_enqueue(cs); ... clocksource_select(); ... And clocksource_enqueue(), makes sure that the list of registered clocksource is always sorted by its rating. And clocksource_select() Chooses best (first in the list) as a clocksource, unless you override It using bootparams or sysfs. Thanks, Vaibhav > > the boot sequence clocksource switch will happen, base on > > kernel parameter (clocksource=). > > > > So logically dmtimer will be always used as a default here. > > Not so at least on my Pandaboard. > > > Thanks, > -- > Ming Lei > -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
