linux-kernel - Re: [PATCH] doc: coresight: correct usage for '/dev/cpu_dma

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Message-ID: <c6d08eff-d92e-9cd2-5c33-1bf67711471c@arm.com>
Date:   Wed, 23 Aug 2017 10:17:06 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Kim Phillips <kim.phillips@....com>
Subject: Re: [PATCH] doc: coresight: correct usage for '/dev/cpu_dma_latency'



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

-- 
Regards,
Sudeep