[RFC PATCH] clocksource: arm_arch_timer: disable the evtstrm via the cmdline

[Catalin Marinas]

On Mon, Jun 20, 2016 at 02:59:11PM +0200, Daniel Lezcano wrote:
> On 06/20/2016 10:21 AM, Will Deacon wrote:
> >On Sun, Jun 19, 2016 at 10:08:38PM +0200, Daniel Lezcano wrote:
> >>On 06/17/2016 03:43 PM, Will Deacon wrote:
> >>
> >>[ Cc'ed tglx ]
> >>
> >>>Disabling the eventstream can be useful for debugging and development
> >>>purposes
> >>
> >>If it is for debugging and development, why upstream this change ?
> >
> >Mainly because it's desirable to be able to debug systems remotely, on
> >machines that you don't have direct access to and where recompiling the
> >kernel isn't necessarily an option. There are plenty of "no*" kernel
> >parameters already that fall into a similar category.
> 
> if the kernel is in development and debug, why this option can't be part of
> debugging code ?

Because we may actually be debugging the hardware rather than the
software. With the event stream enabled, WFE is woken up periodically.
This can be a handy feature for user locking primitives or a simple
workaround for hardware bugs (and we've seen them before). But the side
effect is that it may be potentially hiding hardware issues.

For hardware testing/validation, you'd want to sometimes disable this
feature to check whether your event generation (usually as a result of
exclusive monitor clearing) is working as expected. It's much easier to
do with a command line option.

> I'm not a big fan of the all the specific driver options for the kernel
> parameters. If there is a real need to disable some parts of a driver, it
> would be much more interesting to write a framework for that and then use it
> from arm_arch_timer, thus giving the other drivers the opportunity to
> provide the same feature.

Well, how many non-ARM timer drivers have an exclusive monitor event
stream feature to make sense for a framework?

-- 
Catalin