linux-kernel - Re: [v3 3/5] coresight: add support for debug module

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Date:   Tue, 21 Mar 2017 09:15:03 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Wei Xu <xuwei5@...ilicon.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        John Stultz <john.stultz@...aro.org>,
        Guodong Xu <guodong.xu@...aro.org>,
        Haojian Zhuang <haojian.zhuang@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, linux-clk@...r.kernel.org,
        Mike Leach <mike.leach@...aro.org>,
        Sudeep Holla <Sudeep.Holla@....com>
Subject: Re: [v3 3/5] coresight: add support for debug module

On Tue, Mar 21, 2017 at 07:47:11PM +0800, Leo Yan wrote:
> On Tue, Mar 21, 2017 at 10:16:45AM +0000, Suzuki K Poulose wrote:
> 
> [...]
> 
> > >>In my opinion booting with "nohlt" on the cmd line is sufficient to
> > >>determine if we should use the driver or not.  That way we also avoid
> > >>declaring yet another sysfs flag, something I really want to avoid.
> > >
> > >Agree.
> > >
> > >I did spend some time to implement coresight core framework to support
> > >debug module, you could see it on:http://termbin.com/k2fj; this also
> > >gives me more sense which is better choice. If declaring another sysfs
> > >flag to support debug module in coresight framework, this lets the
> > >codes and interfaces more complex. E.g. for best fit into coresight
> > >framework, finally we can get 8 sysfs nodes for 8 CPUs in system; so
> > >that means we need enable every CPU one by one.
> > 
> > Having a node for each debug area indeed doesn't look good. We could
> > as will stick a single node under /sys/kernel/debug/ which would enable/disable
> > the debug component.
> > 
> > I am OK with it being tied to nohlt. In that case we will have to add
> > a Kconfig dependency on GENERIC_IDLE_POLL_SETUP (though it is selected
> > by default on ARM/ARM64). Parsing the boot command line for nohlt doesn't
> > look like a good idea. We may have to figure out a way to do that.
> 
> I remembered that we can use QoS constraint for CPUIdle:
>         pm_qos_add_request(pm_qos_req, PM_QOS_CPU_DMA_LATENCY, 0);
> 
> Using this way we can disable all low power states and don't depend on
> nohlt anymore.

The idea is to use "nohlt" as a trigger option.  If it is set then we use this
driver - if it isn't then it is kept under wrap.

> 
> > Also, please could you add support for building this as a module ? Since it
> > doesn't depend on the coresight bus anyway, it should be pretty straight
> > forward.
> 
> Yeah. Will support module building.
> 
> Thanks,
> Leo Yan