[PATCH 00/17] coresight: next v4.12-rc4

[Suzuki K Poulose]

On 15/06/17 14:40, Kim Phillips wrote:
> On Thu, 15 Jun 2017 14:08:09 +0100
> Suzuki K Poulose <Suzuki.Poulose at arm.com> wrote:
>
>> On 14/06/17 07:22, Leo Yan wrote:
>>> Hi Kim,
>>>
>>> On Tue, Jun 13, 2017 at 04:17:12PM -0500, Kim Phillips wrote:
>>>
>>> [...]
>>>
>>>>>> Also, do the current juno platforms not have CPU debug modules?  I'd
>>>>>> like to test the new driver.
>>>>>
>>>>> By all means - Leo pointed out the patch adding the CPU debug entries in
>>>>> the DT  file for Juno.
>>>>
>>>> Thanks, I applied it, was able to
>>>> toggle /sys/kernel/debug/coresight_cpu_debug/enable, but wasn't sure
>>>> how to trigger the cpu debug output...
>>>
>>> You could trigger panic flow with command, then can see CPU debug output:
>>> echo c > /proc/sysrq-trigger
>>
>> I think, may be we should provide a handle to trigger the CPU debug dump
>> manually, rather than using it only for panic. Something like :
>>
>> echo 1 > /sys/kernel/debug/coresight_cpu_debug/trigger
>>
>> Thoughts ?
>
> what's the use-case?  I just wanted to do a simple test, and although
> it halted my machine, the sysrq-trigger switch did the job.

Get the CPU dump without crashing the system, when the CPU is looping
indefinitely (with interrupts disabled) ? sysrq-t doesn't tell you where
the task/CPU is at if it is in running state. This can augment the details
by providing the PC. Sure, you can also do this by crashing the system
as above. But it doesn't hurt to provide the same without the crash.


>
> I'm personally not a huge fan of adding more switches, rather for doing
> the right thing on behalf of the user in the first place.  E.g., in the
> case of cpu debug, if configured =y, I don't think it would necessarily
> harm the simple user to have it always be enabled, at boot time and
> beyond, thereby doing away with the .../coresight_cpu_debug/enable

The reasoning behind this is to avoid keeping the debug power domain on
until someone really wants it enabled, so that one can use a single kernel
image in production/development and be able to switch things on at runtime.

> switch and kernel command line option.  If there are users that want to
> turn it on and off dynamically, that can be done by loading and
> unloading the module.

Sure, if someone is building it as a module, they have the luxury. But
this may not be always useful, e.g, crashes at early boot.

Suzuki