• Vladimir Oltean's avatar
    net: dsa: sja1105: configure the PTP_CLK pin as EXT_TS or PER_OUT · 747e5eb3
    Vladimir Oltean authored
    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: default avatarVladimir Oltean <vladimir.oltean@nxp.com>
    Acked-by: default avatarRichard Cochran <richardcochran@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    747e5eb3
sja1105_spi.c 17.5 KB