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

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Date:   Tue, 21 Mar 2017 10:16:45 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     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 21/03/17 02:59, Leo Yan wrote:
> On Mon, Mar 20, 2017 at 10:40:00AM -0600, Mathieu Poirier wrote:
>
> [...]
>
>>>>> If we don't check for "nohlt" some platform may freeze, others may work.
>>>>> If we
>>>>> mandate that "nohlt" be present on the kernel cmd line it works in all
>>>>> cases.
>>>>> As such mandating that "nohlt" be present is a better way to go.
>>>>
>>>>
>>>> Sure, so I will add below checking code in the probe function, please
>>>> let me know if you have alter better way to implement this:
>>>>
>>>> +       if (IS_ENABLED(CONFIG_CPU_IDLE) &&
>>>> +                       !strstr(boot_command_line, "nohlt")) {
>>>> +               dev_err(dev, "May not be accessible in CPU power
>>>> domain.\n");
>>>> +               return -EPERM;
>>>> +       }
>>>>
>>>
>>> There is an API which kind of achieves what "nohlt" does at runtime :
>>>
>>>   cpu_idle_poll_ctrl(true)
>>>
>>> So may be we could use that instead of depending on "nohlt". The other side
>>> of the issues is "when do we decide to use the API". May be we could add
>>> something
>>> like : enable_debug, which could then trigger the panic notifier
>>> registrations
>>> and the above. That would still leave us with a case where the system
>>> crashes
>>> even before the user gets a terminal. May be the following is the best
>>> option :
>>>
>>> 1) Dedicated kernel command line parameter for enabling the CPU debug at
>>> boot/probe.
>>>
>>> and
>>>
>>> 2) Runtime enable method via sysfs.
>>>
>>> What do you think ?
>>
>> 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. 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.

Suzuki