From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Christopher Hall" Subject: Re: [PATCH v4 1/4] Produce system time from correlated clocksource Date: Wed, 14 Oct 2015 18:57:33 -0700 Message-ID: References: <1444675522-4198-1-git-send-email-christopher.s.hall@intel.com> <1444675522-4198-2-git-send-email-christopher.s.hall@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15; format=flowed; delsp=yes Content-Transfer-Encoding: 7bit Cc: jeffrey.t.kirsher@intel.com, hpa@zytor.com, mingo@redhat.com, john.stultz@linaro.org, peterz@infradead.org, x86@kernel.org, intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kevin.b.stanton@intel.com To: "Thomas Gleixner" Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Thomas, On Tue, 13 Oct 2015 12:42:52 -0700, Thomas Gleixner 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