On Tue, Sep 19, 2017 at 03:32:54PM -0600, Mathieu Poirier wrote: > 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 <<< Very appreciate your rephasing and reviewing. Will spin a new patch with it. Thanks, Leo Yan > > + > > +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
