> -----Original Message-----
> From: [email protected] [mailto:linux-omap-
> [email protected]] On Behalf Of Jean Pihet
> Sent: Friday, August 27, 2010 3:17 PM
> To: [email protected]
> Cc: [email protected]; [email protected]
> Subject: Re: [PATCH] OMAP CPUIDLE: CPU Idle latency measurement
>
> Hi,
>
> On Sat, Aug 28, 2010 at 12:08 AM, <[email protected]> wrote:
> > From: Vishwanath BS <[email protected]>
> >
> > This patch has instrumentation code for measuring latencies for
> > various CPUIdle C states for OMAP. Idea here is to capture the
> > timestamp at various phases of CPU Idle and then compute the sw
> > latency for various c states. For OMAP, 32k clock is chosen as
> > reference clock this as is an always on clock. wkup domain memory
> > (scratchpad memory) is used for storing timestamps. One can see the
> > worstcase latencies in below sysfs entries (after enabling
> > CONFIG_CPU_IDLE_PROF
> > in .config). This information can be used to correctly configure cpu idle
> > latencies for various C states after adding HW latencies for each of
> > these sw latencies.
> > /sys/devices/system/cpu/cpu0/cpuidle/state<n>/actual_latency
> > /sys/devices/system/cpu/cpu0/cpuidle/state<n>/sleep_latency
> > /sys/devices/system/cpu/cpu0/cpuidle/state<n>/wkup_latency
> >
> > THis patch is tested on OMAP ZOOM3 using kevin's pm branch.
> >
> > Signed-off-by: Vishwanath BS <[email protected]>
> > Cc: [email protected]
> > ---
> ...
>
> >
> > +#ifdef CONFIG_CPU_IDLE_PROF
> > + sleep_time = omap3_sram_get_sleep_time();
> > + wkup_time = omap3_sram_get_wkup_time();
> > +
> > + /* take care of overflow */
> > + if (postidle_time < preidle_time)
> > + postidle_time += (u32) 0xffffffff;
> > + if (wkup_time < sleep_time)
> > + wkup_time += (u32) 0xffffffff;
> > +
> > + idle_time = postidle_time - preidle_time;
> > + total_sleep_time = wkup_time - sleep_time;
> > + latency = idle_time - total_sleep_time;
> > + sleep_time = omap3_sram_get_sleep_time();
> > + wkup_time = omap3_sram_get_wkup_time();
> Is it needed to re-read the sleep_time and wkup_time values from the
> scratchpad?
Sleep and wake up time stamps were taken just before and after executing wfi
and stored in scratchpad wkup memory. These values are used while computing
actual latency.
> What about the 32k timer overflow? Could that cause the sleep_latency
> and/or wkup_latency to be <0?
Overflow issues are taken care while computing idle_time and total_sleep_time
in code below.
+ if (postidle_time < preidle_time)
+ postidle_time += (u32) 0xffffffff;
+ if (wkup_time < sleep_time)
+ wkup_time += (u32) 0xffffffff;
>
> > +
> > + /* calculate average latency after ignoring sprious ones */
> > + if ((total_sleep_time > 0) && (latency > state->actual_latency)
> > + && (latency >= 0)) {
> > + state->actual_latency = CONVERT_32K_USEC(latency);
> > + latency = (sleep_time - preidle_time);
> Risk of overflow?
Yes I just realized that overflow can cause sleep_latency and wkup_latency to
turn negative. Thanks for pointing it. Will fix it in next version.
>
> > + state->sleep_latency = CONVERT_32K_USEC(latency);
> > + latency = postidle_time - wkup_time;
> Risk of overflow?
Agreed. Will fix it.
Vishwa
>
> > + state->wkup_latency = CONVERT_32K_USEC(latency);
> > + }
> > +#endif
> > +
> > return ts_idle.tv_nsec / NSEC_PER_USEC + ts_idle.tv_sec *
> > USEC_PER_SEC;
> > }
> >
>
> Jean
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