[Intel-gfx] [PATCH 4/6] time: Expose current clocksource in use by timekeeping framework

[Thomas Gleixner]

On Thu, 16 Mar 2017, sourab.gupta at intel.com wrote:

Please Cc LKML for patches which touch subsystems managed on LKML. Also
CC'ing people on the 0/N letter would be helpful, so we can see what this
is about.
 
> For the drivers to be able to use the cross timestamp framework,
> they need the information of current clocksource being used by the
> kernel timekeeping.

No.

> This is needed since the callback given by driver into the
> get_device_system_crosststamp(), in order to synchronously read the
> device time and system counter value, requires the knowledge of the
> clocksource being used to read system counter value (as a part of struct
> system_counterval_t).

Again you are saying WHAT you want but not WHY. The device callback does
not need to know anything about the timekeeping clocksource.

get_device_system_crosststamp() is for timestamps taken via a clock which
is directly correlated with the timekeeper clocksource.

ART and TSC are correlated via:

    TSC = (ART * scale) + offset

get_device_system_crosststamp() invokes the device function which reads
ART, which is converted to CLOCK_MONOTONIC_RAW by the conversion above, and
then uses interpolation to map the CLOCK_MONOTONIC_RAW value to
CLOCK_MONOTONIC.

The device function does not know anything about TSC. All it knows about is
ART.

I have no idea what you are trying to do, but given the explanation above
it's wrong.

Thanks,

	tglx