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

[Suzuki K Poulose]

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