On 15 September 2017 at 04:16, Leo Yan <leo.yan@xxxxxxxxxx> 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@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 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
