[PATCH v3] doc: coresight: correct usage for disabling idle states

[leo.yan@linaro.org (Leo Yan)]

On Mon, Oct 02, 2017 at 11:14:46AM -0700, Mathieu Poirier wrote:
> On 19 September 2017 at 21:46, Leo Yan <leo.yan@linaro.org> wrote:
> > In the coresight CPU debug document it suggests to use 'echo' command
> > to set latency request to /dev/cpu_dma_latency so can disable all CPU
> > idle states, but in fact this doesn't work.
> >
> > This is because when the command 'echo' exits, it releases the device
> > node's file descriptor and the kernel release function removes the QoS
> > constraint; finally when the command 'echo' finished there have no
> > constraint imposed on cpu_dma_latency.
> >
> > This patch changes to use 'exec' to access '/dev/cpu_dma_latency', the
> > command 'exec' can avoid the file descriptor to be closed so we can
> > keep the constraint on cpu_dma_latency.
> >
> > This patch also adds the info for reference docs for PM QoS and cpuidle
> > sysfs.
> >
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Sudeep Holla <sudeep.holla@arm.com>
> > Reported-by: Kim Phillips <kim.phillips@arm.com>
> > Suggested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> >  Documentation/trace/coresight-cpu-debug.txt | 22 +++++++++++++++++-----
> >  1 file changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt
> > index b3da1f9..2b9b51c 100644
> > --- a/Documentation/trace/coresight-cpu-debug.txt
> > +++ b/Documentation/trace/coresight-cpu-debug.txt
> > @@ -149,11 +149,23 @@ If you want to limit idle states at boot time, you can use "nohlt" or
> >
> >  At the runtime you can disable idle states with below methods:
> >
> > -Set latency request to /dev/cpu_dma_latency to disable all CPUs specific idle
> > -states (if latency = 0uS then disable all idle states):
> > -# echo "what_ever_latency_you_need_in_uS" > /dev/cpu_dma_latency
> > -
> > -Disable specific CPU's specific idle state:
> > +It is possible to disable CPU idle states by way of the PM QoS
> > +subsystem, more specifically by using the "/dev/cpu_dma_latency"
> > +interface (see Documentation/power/pm_qos_interface.txt for more
> > +details).  As specified in the PM QoS documentation the requested
> > +parameter will stay in effect until the file descriptor is released.
> > +For example:
> > +
> > +# exec 3<> /dev/cpu_dma_latency; echo 0 >&3
> > +...
> > +Do some work...
> > +...
> > +# exec 3<>-
> > +
> > +The same can also be done from an application program.
> > +
> > +Disable specific CPU's specific idle state from cpuidle sysfs (see
> > +Documentation/cpuidle/sysfs.txt):
> >  # echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable
> >
> 
> Applied.

Thanks a lot, Mathieu.

> Thanks,
> Mathieu
> 
> >
> > --
> > 2.7.4
> >