Long latencies in spidev device driver break real-time performance on Panda/Beagle

Long latencies in spidev device driver break real-time performance on Panda/Beagle

[wally bkg]

I've been trying to run a real-time process to feed a D/A chip using the
spidev driver but am finding the call to /dev/spidevX.Y is responsible for
"random" very long latencies that do not occur if I call the "driver" to
toggle a GPIO pin instead.

My most recent attempts have been with Robert Nelson's 3.9-rc6 kernel for
the Pandaboard ES and he has suggested I contact the OMAP and SPI
maintainers, hence I am sending this message.

Details are here:
http://eewiki.net/display/linuxonarm/PandaBoard_Comments?focusedCommentId=13599653&#comment-13599653

A sample test program that will illustrate the issue can be downloaded
here:
http://eewiki.net/download/attachments/12059008/swave_spidev.c?version=1&modificationDate=1365778735227&api=v2


Robert has run this test code (after changing the GPIO and spidev name
strings for the Beagleboard) and has found the xM is much closer to usable,
but what is curious is that the worst case and number of "missed samples"
are made worse when running as root.

Hoping you have a solution.
--wally.
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