[PATCH v2] doc: coresight: correct usage for disabling idle states

[Leo Yan]

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@xxxxxxx>
Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
Reported-by: Kim Phillips <kim.phillips@xxxxxxx>
Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 Documentation/trace/coresight-cpu-debug.txt | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt
index b3da1f9..205ff95 100644
--- a/Documentation/trace/coresight-cpu-debug.txt
+++ b/Documentation/trace/coresight-cpu-debug.txt
@@ -149,11 +149,15 @@ 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:
+By using PM QoS interface '/dev/cpu_dma_latency', we can set latency
+constraint to disable all CPUs specific idle states (see
+Documentation/power/pm_qos_interface.txt, section 'From user mode');
+below is one example to set latency constraint to '00000000', it is
+hexadecimal format with microsecond unit:
+# exec 3<> /dev/cpu_dma_latency; echo '00000000' >&3
+
+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
 
 
-- 
2.7.4