[PATCH] doc: coresight: correct usage for '/dev/cpu_dma_latency'

Hi Sudeep,

On Wed, Aug 23, 2017 at 10:17:06AM +0100, Sudeep Holla wrote:

On 23/08/17 08:23, Leo Yan wrote:

quoted

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 corrects the description when set latency = 0uS, in
this case the idle state0 still can be selected by CPUIdle governor so
this means on ARM platform the 'WFI' state is still enabled, but all
other deeper states have been disabled so CPUs will not be powered off.

IMO, we should just refer to cpuidle and PM QoS documents from here so
that any updates or changes in those documents are observed. Having a
copy of the text may result in it getting obsolete.

Agree, we should point to below docs:
Documentation/power/pm_qos_interface.txt, section 'From user mode';
Documentation/cpuidle/sysfs.txt;

Thanks for good suggestion.

-- 
Regards,
Sudeep