From 24f0d316fd14c8e61592f580e0075dd3433fde40 Mon Sep 17 00:00:00 2001 From: Leo Yan Date: Tue, 10 Oct 2017 14:32:13 -0600 Subject: doc: coresight: correct usage for disabling idle states 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 Cc: Sudeep Holla Reported-by: Kim Phillips Suggested-by: Mathieu Poirier Signed-off-by: Leo Yan Signed-off-by: Mathieu Poirier Signed-off-by: Greg Kroah-Hartman --- Documentation/trace/coresight-cpu-debug.txt | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) (limited to 'Documentation/trace') diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt index b3da1f90b861..2b9b51cd501e 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 -- cgit v1.2.3