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

Christopher Hall
2015-10-15T01:57:36Z

Thomas,

On Tue, 13 Oct 2015 12:42:52 -0700, Thomas Gleixner <tglx@linutronix.de>
wrote:
> On Mon, 12 Oct 2015, Christopher S. Hall wrote:
>> audio.
>
> This wants to be a seperate patch, really.

OK. This makes sense, I'll do this the next time.

>> +/* This needs to be 3 or greater for backtracking to be useful */
>
> Why?

The current index points to a copy and the next may be being changed by
update_wall_time(). Leaving n-2 entries available with useful history in
them. I'll add more descriptive comments here.

>
>> +#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.

>> +static int shadow_index = -1; /* incremented to zero in
>
> What's the point of this? Aside of that, please do not use tail comments.

It's removed. A check for validity is added below and this isn't
necessary.

> That's silly. Make DEPTH a power of 2 and do:
>
> idx = (idx + 1) & (DEPTH - 1);

This is changed.

>> + true : *shadow_index_out < shadow_index;
>
> All this can go away.

Yes.

>> + /* Also make sure that entry is valid based on current shadow_index */
>> + *shadow_index_io = ret;
>> + return true;
>
> You surely try hard to do stuff in the most unreadable way.

Is like this easier to follow?

+static struct timekeeper *search_shadow_history(cycles_t cycles,
+ struct clocksource *cs)
+{
+ struct timekeeper *tk = &tk_core.timekeeper;
+ int srchidx = shadow_index;
+ cycles_t cycles_start, cycles_end;
+
+ cycles_start = tk->tkr_mono.cycle_last;
+ do {
+ srchidx = !srchidx-- ? srchidx+SHADOW_HISTORY_DEPTH :
srchidx;
+ tk = shadow_timekeeper + srchidx;
+
+ /* The next shadow entry may be in flight, don't use it */
+ if (srchidx == ((shadow_index+1) &
(SHADOW_HISTORY_DEPTH-1)))
+ return NULL;
+
+ /* Make sure timekeeper is related to clock on this
interval */
+ if (tk->tkr_mono.clock != cs)
+ return NULL;
+
+ cycles_end = cycles_start;
+ cycles_start = tk->tkr_mono.cycle_last;
+ } while (!cycle_between(cycles_start, cycles, cycles_end));
+
+ return tk;
+}
A check for validity is added here using the clocksource pointer.

and inside of get_correlated_timestamp():

+ * into account. If the value is in the past, try to
backtrack
+ */
+ cycles_end = tk->tkr_mono.read(tk->tkr_mono.clock);
+ cycles_start = tk->tkr_mono.cycle_last;
+ if (!cycle_between(cycles_start, cycles, cycles_end)) {
+ tk = search_shadow_history(cycles,
crs->related_cs);
+ if (!tk)
+ return -EAGAIN;
+ }

>> + /*
>> + * Get a timestamp from the device if get_ts is non-NULL
>> + */
>> + if( crt->get_ts ) {
>> + ret = crt->get_ts(crt);
>> + if (ret)
>> + return ret;
>> + }
>
> What's the point of this? Why are you not making the few lines which
> you can actually reuse a helper function and leave the PTP code alone?

The audio driver is structured in such a way that it's simpler to provide
a value rather than a callback. I changed this to allow the audio
developers to provide an ART value as input. If a callback is provided,
the resulting counter value is guaranteed to be later than cycle_last and
there is no need to do extra checking (the goto skips that check). Is
this an answer to your question?

> So I reached enf of patch and did not find anything in
> timekeeping_init() which tells that the index is incremented to 0. It
> really would need a comment, but why do you want to do that at all. It
> does not matter whether the first entry is at 0 or 1. You need a
> validity check for the entries anyway.

I think this should be resolved. There's no sensitivity with regard to
the start index with an added validity check.

Thanks,
Chris