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

[Mathieu Poirier]

On 15 September 2017 at 04:16, Leo Yan <leo.yan at 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 at lwn.net>
> Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
> Cc: Sudeep Holla <sudeep.holla at arm.com>
> Reported-by: Kim Phillips <kim.phillips at arm.com>
> Signed-off-by: Leo Yan <leo.yan at linaro.org>
> ---
>  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

Since doing echo '00000000' >&3 or simply echo 0 >&3 yields the same
result I would go for the latter.  I also think it is important to
specify that using an "echo" command without holding the file open
won't give the desired result.  I would reformat your paragraph as
follow:

>>> Begin >>>

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.

<<< End <<<


> +
> +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
>