Re: [PATCH v4 1/4] Produce system time from correlated clocksource

Christopher Hall
2015-10-20T00:18:41Z

Thomas,

On Thu, 15 Oct 2015 01:15:57 -0700, Thomas Gleixner <tglx@linutronix.de>
wrote:
>> >
>> > > +#define SHADOW_HISTORY_DEPTH 7
>> >
>> > And that number is 7 because?
>>
>> Due to power of 2 it will be 8 instead. As above the useful history is
>> 8-2*1
>> ms (1 ms is the minimum jiffy length). Array size 4 would not be enough
>> history for the DSP which requires 4 ms of history, in the worst case.
>
> And how exactly becomes 7 magically 8?

I'm making the array size a power of two, per your suggestion.

In my view, the candidate array sizes are 4 and 8. But the DSP driver
code requires that it be at least 8. Here is my reasoning:

The number of shadow_timekeeper array elements that contain useful history
is n-2 where n is the size of the shadow_timekeeper array. This is true
because shadow_timekeeper[shadow_index] is a copy of tk_core.timekeeper
(this isn't history). The next entry of the shadow_timekeeper array may
be in-flight and contain invalid information, because update_wall_time()
makes changes to the next entry of shadow timekeeper outside of the
sequence lock. If that occurs, get_correlated_timestamp() would not be
notified of this change through a change in sequence number.

Combining these two requirements, the candidate array sizes and their
effective history sizes is:

Array Size Effective History (in elements)
========== ===============================
4 2
8 6
16 14

update_wall_clock() checks that the number of elapsed cycles is >=
tk->cycle_interval before performing any updates to shadow_timekeeper.
The minimum cycle count (and minimum time) that is contained in each
history element is equal to cycle_interval which represents a time period
of 1/HZ seconds. The smallest configurable period for cycle_interval is 1
ms (in cycles). Each history element, therefore, is guaranteed to contain
1 ms of history for all kernel configurations. For correct operation of
the DSP using the proposed ART->system time API we need at least 4 ms of
history. To guarantee this much history is available, the array size
needs to be 8.

Array Size Minimum Effective History (in ms)
========== =================================
4 2
8 6 <----
16 14

Does this make sense for the choice of array size 8?

> Make it a seperate function which can hand in the information and
> leave the PTP specific sample/conversion function alone.

OK. The audio driver code will conform to the original correlated
clocksource API.

FYI, I will be away from work and email until 10/28. Any responses will
be delayed until then.

Thanks,
Chris