From mboxrd@z Thu Jan  1 00:00:00 1970
From: leo.yan@linaro.org (Leo Yan)
Date: Thu, 15 Jun 2017 23:50:54 +0800
Subject: [PATCH 00/17] coresight: next v4.12-rc4
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 <20170613161712.f1f3cac734939d8728c7ed77@arm.com>
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Message-ID: <20170615155054.GN28496@leoy-ThinkPad-T440>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Hi Kim,

On Thu, Jun 15, 2017 at 09:22:46AM -0500, Kim Phillips wrote:
> On Thu, 15 Jun 2017 09:41:22 +0100
> Sudeep Holla <sudeep.holla@arm.com> wrote:
> 
> > On 15/06/17 04:34, Kim Phillips wrote:
> > [...]
> > > How does one tell whether writes to /dev/cpu_dma_latency are effective?
> > > 
> > Watch /sys/devices/system/cpu/cpu*/cpuidle/state*/{usage,time}, they
> > shouldn't tick based on the value set to cpu_dma_latency.
> > 
> > E.g. a value of 0 should stop entering all the idle states.
> 
> I see all states following the same pattern whether 0 or not:
> 
> root at juno:~# cat show2.sh 
> show()
> {
> 	echo $1 > /dev/cpu_dma_latency
> 	echo === $1:
> 	cat /sys/devices/system/cpu/cpu2/cpuidle/state*/{usage,time}
> 	echo ---
> 	sleep 10
> 	cat /sys/devices/system/cpu/cpu2/cpuidle/state*/{usage,time}
> }
> 
> show 0
> show 1500
> show 0
> root at juno:~# taskset -c 2 bash ./show2.sh
> === 0:
> 1893
> 894
> 5417
> 1398675
> 65997648
> 3818880750
> ---
> 1895
> 905
> 5434
> 1401721
> 68076867
> 3826795502
> === 1500:
> 1895
> 905
> 5434
> 1401721
> 68076867
> 3826795502
> ---
> 1895
> 918
> 5447
> 1401721
> 69403510
> 3835466081
> === 0:
> 1895
> 918
> 5447
> 1401721
> 69403510
> 3835466081
> ---
> 1896
> 929
> 5463
> 1403577
> 71454839
> 3843409979
> 
> googling suggests the /dev/cpu_dma_latency file should be *kept* open,
> and written a 32-bit binary number:
> 
> https://access.redhat.com/articles/65410
> 
> If so (I don't have time to test), this cpu debug documentation is very
> deceptive.

Looked into the code, and I must apologize I have not verified the
command when wrote the doc. As you pointed out, the closing file
operation releases the constraint for PM QoS.

Below command can work (I tested):
exec 3<> /dev/cpu_dma_latency; echo 0 >&3

Will commit one patch for this.

Thanks,
Leo Yan