af9fdd2bf8
The current poll interval is enough to ensure that rising and falling edge events are not lost for a 1 PPS signal with 50% duty cycle. But when we deliver the events to user space, it will try to infer if they were corresponding to a rising or to a falling edge (the kernel driver doesn't know that either). User space will try to make that inference based on the time at which the PPS master had emitted the pulse (i.e. if it's a .0 time, it's rising edge, if it's .5 time, it's falling edge). But there is no in-kernel API for retrieving the precise timestamp corresponding to a PPS master (aka perout) pulse. So user space has to guess even that. It will read the PTP time on the PPS master right after we've delivered the extts event, and declare that the PPS master time was just the closest integer second, based on 2 thresholds (lower than .25, or higher than .75, and ignore anything else). Except that, if we poll for extts events (and our hardware doesn't really help us, by not providing an interrupt), then there is a risk that the poll period (and therefore the time at which the event is delivered) might confuse user space. Because we are always scheduling the next extts poll at SJA1105_EXTTS_INTERVAL "from now" (that's the only thing that the schedule_delayed_work() API gives us), it means that the start time of the next delayed workqueue will always be shifted to the right a little bit (shifted with the SPI access duration of this workqueue run). In turn, because user space sees extts events that are non-periodic compared to the PPS master's time, this means that it might start making wrong guesses about rising/falling edge. To understand the effect, here is the output of ts2phc currently. Notice the 'src' timestamps of the 'SKIP extts' events, and how they have a large wander. They keep increasing until the upper limit for the ignore threshold (.75 seconds), after which the application starts ignoring the _other_ edge. ts2phc[26.624]: /dev/ptp3 SKIP extts index 0 at 21.449898912 src 21.657784518 ts2phc[27.133]: adding tstamp 21.949894240 to clock /dev/ptp3 ts2phc[27.133]: adding tstamp 22.000000000 to clock /dev/ptp1 ts2phc[27.133]: /dev/ptp3 offset 640 s2 freq +5112 ts2phc[27.636]: /dev/ptp3 SKIP extts index 0 at 22.449889360 src 22.669398022 ts2phc[28.140]: adding tstamp 22.949884376 to clock /dev/ptp3 ts2phc[28.140]: adding tstamp 23.000000000 to clock /dev/ptp1 ts2phc[28.140]: /dev/ptp3 offset 96 s2 freq +4760 ts2phc[28.644]: /dev/ptp3 SKIP extts index 0 at 23.449879504 src 23.677420422 ts2phc[29.153]: adding tstamp 23.949874704 to clock /dev/ptp3 ts2phc[29.153]: adding tstamp 24.000000000 to clock /dev/ptp1 ts2phc[29.153]: /dev/ptp3 offset -264 s2 freq +4429 ts2phc[29.656]: /dev/ptp3 SKIP extts index 0 at 24.449870008 src 24.689407238 ts2phc[30.160]: adding tstamp 24.949865376 to clock /dev/ptp3 ts2phc[30.160]: adding tstamp 25.000000000 to clock /dev/ptp1 ts2phc[30.160]: /dev/ptp3 offset -280 s2 freq +4334 ts2phc[30.664]: /dev/ptp3 SKIP extts index 0 at 25.449860760 src 25.697449926 ts2phc[31.168]: adding tstamp 25.949856176 to clock /dev/ptp3 ts2phc[31.168]: adding tstamp 26.000000000 to clock /dev/ptp1 ts2phc[31.168]: /dev/ptp3 offset -176 s2 freq +4354 ts2phc[31.672]: /dev/ptp3 SKIP extts index 0 at 26.449851584 src 26.705433606 ts2phc[32.180]: adding tstamp 26.949846992 to clock /dev/ptp3 ts2phc[32.180]: adding tstamp 27.000000000 to clock /dev/ptp1 ts2phc[32.180]: /dev/ptp3 offset -80 s2 freq +4397 ts2phc[32.684]: /dev/ptp3 SKIP extts index 0 at 27.449842384 src 27.717415110 ts2phc[33.192]: adding tstamp 27.949837768 to clock /dev/ptp3 ts2phc[33.192]: adding tstamp 28.000000000 to clock /dev/ptp1 ts2phc[33.192]: /dev/ptp3 offset 0 s2 freq +4453 ts2phc[33.696]: /dev/ptp3 SKIP extts index 0 at 28.449833128 src 28.729412902 ts2phc[34.200]: adding tstamp 28.949828472 to clock /dev/ptp3 ts2phc[34.200]: adding tstamp 29.000000000 to clock /dev/ptp1 ts2phc[34.200]: /dev/ptp3 offset 8 s2 freq +4461 ts2phc[34.704]: /dev/ptp3 SKIP extts index 0 at 29.449823816 src 29.737416038 ts2phc[35.208]: adding tstamp 29.949819152 to clock /dev/ptp3 ts2phc[35.208]: adding tstamp 30.000000000 to clock /dev/ptp1 ts2phc[35.208]: /dev/ptp3 offset -8 s2 freq +4447 ts2phc[35.712]: /dev/ptp3 SKIP extts index 0 at 30.449814496 src 30.745554982 ts2phc[36.216]: adding tstamp 30.949809840 to clock /dev/ptp3 ts2phc[36.216]: adding tstamp 31.000000000 to clock /dev/ptp1 ts2phc[36.216]: /dev/ptp3 offset -8 s2 freq +4445 ts2phc[36.468]: /dev/ptp3 SKIP extts index 0 at 31.449805184 src 31.501109446 ts2phc[36.972]: adding tstamp 31.949800536 to clock /dev/ptp3 ts2phc[36.972]: adding tstamp 32.000000000 to clock /dev/ptp1 ts2phc[36.972]: /dev/ptp3 offset -8 s2 freq +4442 ts2phc[37.480]: /dev/ptp3 SKIP extts index 0 at 32.449795896 src 32.513320070 ts2phc[37.984]: adding tstamp 32.949791248 to clock /dev/ptp3 ts2phc[37.984]: adding tstamp 33.000000000 to clock /dev/ptp1 ts2phc[37.984]: /dev/ptp3 offset 0 s2 freq +4448 Fix that by taking the following measures: - Schedule the poll from a timer. Because we are really scheduling the timer periodically, the extts events delivered to user space are periodic too, and don't suffer from the "shift-to-the-right" effect. - Increase the poll period to 6 times a second. This imposes a smaller upper bound to the shift that can occur to the delivery time of extts events, and makes user space (ts2phc) to always interpret correctly which events should be skipped and which shouldn't. - Move the SPI readout itself to the main PTP kernel thread, instead of the generic workqueue. This is because the timer runs in atomic context, but is also better than before, because if needed, we can chrt & taskset this kernel thread, to ensure it gets enough priority under load. After this patch, one can notice that the wander is greatly reduced, and that the latencies of one extts poll are not propagated to the next. The 'src' timestamp that is skipped is never larger than .65 seconds (which means .15 seconds larger than the time at which the real event occurred at, and .10 seconds smaller than the .75 upper threshold for ignoring the falling edge): ts2phc[40.076]: adding tstamp 34.949261296 to clock /dev/ptp3 ts2phc[40.076]: adding tstamp 35.000000000 to clock /dev/ptp1 ts2phc[40.076]: /dev/ptp3 offset 48 s2 freq +4631 ts2phc[40.568]: /dev/ptp3 SKIP extts index 0 at 35.449256496 src 35.595791078 ts2phc[41.064]: adding tstamp 35.949251744 to clock /dev/ptp3 ts2phc[41.064]: adding tstamp 36.000000000 to clock /dev/ptp1 ts2phc[41.064]: /dev/ptp3 offset -224 s2 freq +4374 ts2phc[41.552]: /dev/ptp3 SKIP extts index 0 at 36.449247088 src 36.579825574 ts2phc[42.044]: adding tstamp 36.949242456 to clock /dev/ptp3 ts2phc[42.044]: adding tstamp 37.000000000 to clock /dev/ptp1 ts2phc[42.044]: /dev/ptp3 offset -240 s2 freq +4290 ts2phc[42.536]: /dev/ptp3 SKIP extts index 0 at 37.449237848 src 37.563828774 ts2phc[43.028]: adding tstamp 37.949233264 to clock /dev/ptp3 ts2phc[43.028]: adding tstamp 38.000000000 to clock /dev/ptp1 ts2phc[43.028]: /dev/ptp3 offset -144 s2 freq +4314 ts2phc[43.520]: /dev/ptp3 SKIP extts index 0 at 38.449228656 src 38.547823238 ts2phc[44.012]: adding tstamp 38.949224048 to clock /dev/ptp3 ts2phc[44.012]: adding tstamp 39.000000000 to clock /dev/ptp1 ts2phc[44.012]: /dev/ptp3 offset -80 s2 freq +4335 ts2phc[44.508]: /dev/ptp3 SKIP extts index 0 at 39.449219432 src 39.535846118 ts2phc[44.996]: adding tstamp 39.949214816 to clock /dev/ptp3 ts2phc[44.996]: adding tstamp 40.000000000 to clock /dev/ptp1 ts2phc[44.996]: /dev/ptp3 offset -32 s2 freq +4359 ts2phc[45.488]: /dev/ptp3 SKIP extts index 0 at 40.449210192 src 40.515824678 ts2phc[45.980]: adding tstamp 40.949205568 to clock /dev/ptp3 ts2phc[45.980]: adding tstamp 41.000000000 to clock /dev/ptp1 ts2phc[45.980]: /dev/ptp3 offset 8 s2 freq +4390 ts2phc[46.636]: /dev/ptp3 SKIP extts index 0 at 41.449200928 src 41.664176902 ts2phc[47.132]: adding tstamp 41.949196288 to clock /dev/ptp3 ts2phc[47.132]: adding tstamp 42.000000000 to clock /dev/ptp1 ts2phc[47.132]: /dev/ptp3 offset 0 s2 freq +4384 ts2phc[47.620]: /dev/ptp3 SKIP extts index 0 at 42.449191656 src 42.648117190 ts2phc[48.112]: adding tstamp 42.949187016 to clock /dev/ptp3 ts2phc[48.112]: adding tstamp 43.000000000 to clock /dev/ptp1 ts2phc[48.112]: /dev/ptp3 offset 0 s2 freq +4384 ts2phc[48.604]: /dev/ptp3 SKIP extts index 0 at 43.449182384 src 43.632112582 ts2phc[49.100]: adding tstamp 43.949177736 to clock /dev/ptp3 ts2phc[49.100]: adding tstamp 44.000000000 to clock /dev/ptp1 ts2phc[49.100]: /dev/ptp3 offset -8 s2 freq +4376 ts2phc[49.588]: /dev/ptp3 SKIP extts index 0 at 44.449173096 src 44.616136774 ts2phc[50.080]: adding tstamp 44.949168464 to clock /dev/ptp3 ts2phc[50.080]: adding tstamp 45.000000000 to clock /dev/ptp1 ts2phc[50.080]: /dev/ptp3 offset 8 s2 freq +4390 ts2phc[50.572]: /dev/ptp3 SKIP extts index 0 at 45.449163816 src 45.600134662 ts2phc[51.064]: adding tstamp 45.949159160 to clock /dev/ptp3 ts2phc[51.064]: adding tstamp 46.000000000 to clock /dev/ptp1 ts2phc[51.064]: /dev/ptp3 offset -8 s2 freq +4376 ts2phc[51.556]: /dev/ptp3 SKIP extts index 0 at 46.449154528 src 46.584588550 ts2phc[52.048]: adding tstamp 46.949149896 to clock /dev/ptp3 ts2phc[52.048]: adding tstamp 47.000000000 to clock /dev/ptp1 ts2phc[52.048]: /dev/ptp3 offset 0 s2 freq +4382 ts2phc[52.540]: /dev/ptp3 SKIP extts index 0 at 47.449145256 src 47.568132198 ts2phc[53.032]: adding tstamp 47.949140616 to clock /dev/ptp3 ts2phc[53.032]: adding tstamp 48.000000000 to clock /dev/ptp1 ts2phc[53.032]: /dev/ptp3 offset 0 s2 freq +4382 ts2phc[53.524]: /dev/ptp3 SKIP extts index 0 at 48.449135968 src 48.552121446 ts2phc[54.016]: adding tstamp 48.949131320 to clock /dev/ptp3 ts2phc[54.016]: adding tstamp 49.000000000 to clock /dev/ptp1 ts2phc[54.016]: /dev/ptp3 offset 0 s2 freq +4382 ts2phc[54.512]: /dev/ptp3 SKIP extts index 0 at 49.449126680 src 49.540147014 ts2phc[55.000]: adding tstamp 49.949122040 to clock /dev/ptp3 ts2phc[55.000]: adding tstamp 50.000000000 to clock /dev/ptp1 ts2phc[55.000]: /dev/ptp3 offset 0 s2 freq +4382 ts2phc[55.492]: /dev/ptp3 SKIP extts index 0 at 50.449117400 src 50.520119078 ts2phc[55.988]: adding tstamp 50.949112768 to clock /dev/ptp3 ts2phc[55.988]: adding tstamp 51.000000000 to clock /dev/ptp1 ts2phc[55.988]: /dev/ptp3 offset 8 s2 freq +4390 ts2phc[56.476]: /dev/ptp3 SKIP extts index 0 at 51.449108120 src 51.504175910 ts2phc[57.132]: adding tstamp 51.949103480 to clock /dev/ptp3 ts2phc[57.132]: adding tstamp 52.000000000 to clock /dev/ptp1 ts2phc[57.132]: /dev/ptp3 offset 0 s2 freq +4384 ts2phc[57.624]: /dev/ptp3 SKIP extts index 0 at 52.449098840 src 52.651833574 ts2phc[58.116]: adding tstamp 52.949094200 to clock /dev/ptp3 ts2phc[58.116]: adding tstamp 53.000000000 to clock /dev/ptp1 ts2phc[58.116]: /dev/ptp3 offset 8 s2 freq +4392 ts2phc[58.612]: /dev/ptp3 SKIP extts index 0 at 53.449089560 src 53.639826918 ts2phc[59.100]: adding tstamp 53.949084920 to clock /dev/ptp3 ts2phc[59.100]: adding tstamp 54.000000000 to clock /dev/ptp1 ts2phc[59.100]: /dev/ptp3 offset 8 s2 freq +4394 ts2phc[59.592]: /dev/ptp3 SKIP extts index 0 at 54.449080272 src 54.619842278 ts2phc[60.084]: adding tstamp 54.949075624 to clock /dev/ptp3 ts2phc[60.084]: adding tstamp 55.000000000 to clock /dev/ptp1 ts2phc[60.084]: /dev/ptp3 offset 8 s2 freq +4397 ts2phc[60.576]: /dev/ptp3 SKIP extts index 0 at 55.449070968 src 55.603885542 ts2phc[61.068]: adding tstamp 55.949066312 to clock /dev/ptp3 ts2phc[61.068]: adding tstamp 56.000000000 to clock /dev/ptp1 ts2phc[61.068]: /dev/ptp3 offset 0 s2 freq +4391 ts2phc[61.560]: /dev/ptp3 SKIP extts index 0 at 56.449061680 src 56.587885798 ts2phc[62.052]: adding tstamp 56.949057032 to clock /dev/ptp3 ts2phc[62.052]: adding tstamp 57.000000000 to clock /dev/ptp1 ts2phc[62.052]: /dev/ptp3 offset -8 s2 freq +4383 Signed-off-by: Vladimir Oltean <olteanv@gmail.com> Acked-by: Richard Cochran <richardcochran@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
||
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README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.