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The SJA1105 switch family has a PTP_CLK pin which emits a signal with fixed 50% duty cycle, but variable frequency and programmable start time. On the second generation (P/Q/R/S) switches, this pin supports even more functionality. The use case described by the hardware documents talks about synchronization via oneshot pulses: given 2 sja1105 switches, arbitrarily designated as a master and a slave, the master emits a single pulse on PTP_CLK, while the slave is configured to timestamp this pulse received on its PTP_CLK pin (which must obviously be configured as input). The difference between the timestamps then exactly becomes the slave offset to the master. The only trouble with the above is that the hardware is very much tied into this use case only, and not very generic beyond that: - When emitting a oneshot pulse, instead of being told when to emit it, the switch just does it "now" and tells you later what time it was, via the PTPSYNCTS register. [ Incidentally, this is the same register that the slave uses to collect the ext_ts timestamp from, too. ] - On the sync slave, there is no interrupt mechanism on reception of a new extts, and no FIFO to buffer them, because in the foreseen use case, software is in control of both the master and the slave pins, so it "knows" when there's something to collect. These 2 problems mean that: - We don't support (at least yet) the quirky oneshot mode exposed by the hardware, just normal periodic output. - We abuse the hardware a little bit when we expose generic extts. Because there's no interrupt mechanism, we need to poll at double the frequency we expect to receive a pulse. Currently that means a non-configurable "twice a second". Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Acked-by: Richard Cochran <richardcochran@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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.