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SubjectRe: [RFC PATCH 01/13] arm: Fix mutual exclusion in arch_gettimeoffset
On Tue, 13 Nov 2018, Russell King - ARM Linux wrote:

> A clocksource provides a cycle counter that monotonically changes and
> does not wrap between clockevent events.
> A clock event is responsible for providing events to the system when
> some work is needing to be done, limited by the wrap interval of the
> clocksource.
> Each time the clock event triggers an interrupt, the clocksource is
> read to determine how much time has passed, using:
> count = (new_value - old_value) & available_bits
> nanosecs = count * scale >> shift;
> If you try to combine the clocksource and clockevent because you only
> have a single counter, and the counter has the behaviour of:
> - counting down towards zero
> - when reaching zero, triggers an interrupt, and reloads with N
> then this provides your clockevent, but you can't use this as a clock
> source, because each time you receive an interrupt and try to read the
> counter value, it will be approximately the same value. This means
> that the above calculation fails to register the correct number of
> nanoseconds passing. Hence, this does not work.
> Also note where I said above that the clock event device must be able
> to provide an interrupt _before_ the clocksource wraps - clearly with
> such a timer, that is utterly impossible.
> The simple solution is to not use such a counter as the clocksource,
> which means you fall back to using the jiffies clocksource, and your
> timekeeping has jiffy resolution - so 10ms, or possibly 1ms if you
> want a 1kHz timer interval. For most applications, that's simply way
> to coarse, as was realised in the very early days of Linux.
> If only there was a way to interpolate between timer interrupts...
> which is exactly what arch_gettimeoffset() does, and is a historical
> reminant of the pre-clocksource/clockevent days of the kernel - but
> it is the only way to get better resolution from this sort of setup.

Both of the platforms in question (RPC and EBSA110) have not
defined(CONFIG_GENERIC_CLOCKEVENTS) and have not defined any struct
clock_event_device, AFAICT.

So, even assuming that you're right about the limitations of single-timer
platforms in general, removal of arch_gettimeoffset wouldn't require the
removal of any platforms, AFAICT.


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