Re: [PATCH v3 3/5] coresight: add support for debug module

[Mike Leach]

On 22 March 2017 at 14:07, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>
>
> On 22/03/17 12:54, Mike Leach wrote:
>>
>>
>> On 21 March 2017 at 15:39, Sudeep Holla <sudeep.holla@xxxxxxx
>> <mailto:sudeep.holla@xxxxxxx>> wrote:
>>
> [...]
>
>> I disagree with this approach. One of the main usefulness of such
>> self hosted debug feature is to debug issues around features like
>> cpuidle. Adding constraints like "cpuidle needs to be disabled" is
>> not good IMO. There are ways to make it work with cpuidle enabled.
>> Please explore them. In particular refer H9.2.39 EDPRCR, External
>> Debug Power/Reset Control Register.
>>
>> So, "nohlt" option is not an option. I prefer some sysfs option like
>> Suzuki suggested to enable this feature on demand if power saving in
>> normal usecase is the concern. Using "nohlt" just disables idle and
>> doesn't ensure the debug power domain is ON. Using the flag directly
>> in this driver to enable debug power domain also sounds misuse of
>> that flag for me.
>>
>> I think the key issue to remember here is that experience with
>> external debug shows that CPU Idle means different things to
>> different SoC designs / power management schemes. (and we are using
>> external debug in a self hosted way here).
>>
>
> Yes agreed on the point that meaning of "cpuidle" differs on each SoC.
>
>> Some designs will power down an entire cluster if all CPUs on the
>> cluster are powered down - including the parts of the debug
>> registers that should remain powered in the debug power domain.
>
> Interesting, at-least ETMv4 or some other coresight specification
> clearly classify the power domains and the register access. The actual
> power domain itself may vary depending on implementation.

Yes - the ETMv4 spec defines what should be in the core / debug power
domains, but there is no architectural requirement for these to be
separate.
Most of power management is "implementation defined", & hw designers
seem to have different criteria than sw engineers wanting to debug
stuff.

>
>> The bits in EDPRCR are not respected in these cases - these designs
>> do not really support debug over power down in the way that the
>> CoreSight / Debug designers anticipated. This means that even
>> checking EDPRSR has the potential to cause a bus hang if the target
>> register is unpowered. (and if the debug power domain is unpowered
>> then the PC data is also lost).
>>
>
> Agreed, but can we start supporting the sane designs in sane way first.
> We can always add compatible and handle deviations. I agree we may need
> to support such deviations but starting with that seems setting a bad
> example.
>

>From a pragmatic point of view, we have to support the designs that we
have and are currently using.
Sadly this might include some that do not behave in an ideal way.

I'm not saying disabling CPUIdle is right for all cases, or perhaps
many, but it has in the past been useful in specific instances - not
just for external debug, but to use CoreSight trace etc, were powering
down the a CPU / cluster takes out ETM accesses and breaks stuff.

The key is that to use this driver, the user has to be aware of the PM
implications on their specific system - the kernel may not take care
of it all for them - as SCP type power controllers are often external
and may have unique firmware and capabilites.

Historically CPUIdle disable has been used as a blunt instrument to
handle power management problems in real debug use cases - but it is
one that has been successful. Where there are better methods then I am
all for using these.


>> In these cases, accessing to the debug registers while they are not
>> powered is a recipe for disaster - so preventing CPUIdle and the
>> subsequent cluster power down allows investigation on this class of
>> system - and allowing the CPUs of interest be interrogated without
>> hanging the crash log process.
>>
>
> Agreed. But my point is that many issues are around cpuidle and some
> usecase and just eliminating that use-case sounds bad. For me,
> core-sight was most useful to debug issues around cpu power management
> and lockups where we can't stop cores but examine these registers.
> There are other alternatives for other use-cases IMO.

For your case, removing what you are interested in debugging is
evidently counter productive, so other techniques need to be used to
ensure the CS regs remain alive.
But equally there could be use cases where this might be just fine or
even the only way.

>
>>
>> On systems that do behave correctly with respect to debug power
>> domains, then disabling CPUIdle is unnecessary - these can be
>> controlled by EDPRCR - perhaps; per the specification it is
>> "implementation defined" if writing bits to this register have an
>> effect on the system anyway even if the debug domain is correctly
>> powered.
>>
>
> We can always do that unconditionally. If implementations don't honor
> those bits, it's different. If they hang on accessing something which is
> on debug power domain and not on core power domain, then you have much
> bigger issue to solve. How can you even trust and make any other
> register accesses that are in debug power domain then ?
>

It is difficult and highly platform dependent. For external debug we
might have per-platform rules built into the debugger on what can and
cannot be done and when, plus on occasion some power management
scripts. Those platforms that get closest to the "standard" CoreSight
power management are easiest to debug.

>> While it is true to say that disabling CPUIdle does not guarantee
>> that the debug power domain is on, it does in a certain class of
>> designs prevent it being powered off (Juno historically - not sure if
>> that is still the case.).
>>
>
> Again it's completely platform specific. All you need to care is that
> the debug power domain is on or not. Disabling CPUIdle to achieve that
> is simply wrong and may work only on few platforms.
>
>> However, I do agree that the use of the driver should not be
>> triggered _only_ on the existence of /nohlt on the command line -
>> there is a class of designs where this will not be required.
>>
>
> Thanks
>
>> When enabing the driver as a kernel config the user needs to
>> decide:- 1) do I need this to debug the issue I am seeing 2) does the
>> power management on my system require I use /nohlt as well.
>
> Please don't *misuse* nohlt to disable idle. There are other ways to
> do the same either from the user-space or from the driver.

I'm not advocating /nohlt above anything else - it's just what is
being discussed here. Furthermore, no one debug technique is ever
going to be appropriate in all circumstances - debug and trace are
always a compromise.

I initially raised the issue of clusters powering down, and
possibility that no CPUIdle might prevent this, to ensure that
awareness is built in to driver / config / help text /documentation
that these are real issues seen in the external debug world.

The key point is that the caveat in using this driver is that the
power management has to be considered on a platform specific basis
before it is configured; and appropriate actions may be needed for it
to work correctly. Without this then the driver could cause more
issues than it debugs. A user selecting this _must_ be told about
these issues

>
>>
>> I think that the use of /nohlt as an option, and the reasons why it
>> might be needed should be part of the configuration help in this
>> case.
>>
>> There is also a case for considering if there should be an option to
>> configure it to be enabled or disabled at boot time. It is easy to
>> imagine cases I want to have this running from the start as a crash
>> happens early - and cases I can enable it on demand later.
>>
>
> Also consider with cpuidle enabled ;). I can help testing if needed.
>
> --
> Regards,
> Sudeep

Regards

Mike

-- 
Mike Leach
Principal Engineer, ARM Ltd.
Blackburn Design Centre. UK