Messages in this thread | | | Subject | Re: [PATCH v4 1/4] Produce system time from correlated clocksource | Date | Mon, 19 Oct 2015 17:18:39 -0700 | From | "Christopher Hall" <> |
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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
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