Interesting info on spidev, but it doesn't surprise me. Isn't that because they are non blocking read/writes to manage slow spi clocks, etc? The calls end up in spi_sync, that says "This call may only be used from a context that may sleep" so I would expect not so good performance with this. My first thought would be a blocking/non sleeping hardware call, although I am not clear if this would require direct register calls to the spi memory locations. With a 10mhz spi ic maybe that would take too long.
On Thu, Oct 13, 2016 at 5:33 PM, Jeff Epler <jep...@unpythonic.net> wrote: > If your device uses PREEMPT-RT realtime, then you can use any userspace > interface (ioctl, read, write, recv, send, etc) that gives you > acceptable latency in practice. Otherwise, you basically get to do > direct memory-mapped control of the applicable registers in your SOC, > because for xenomai and rtai you can't use standard kernel or syscall > interfaces in realtime context (but you can for setup/cleanup code, for > instance to get actually map MMIO regions) > > The SPI hardware interface, with /dev/spidev as a userspace API, is > perfect on paper; however, I have experenced that the quality of the > driver implementation is often poor, both in latency and in, well, > actually working! > > Here are some notes about various ARM SBCs and SPI interface hardware. > Except for odroid c2, these are all my direct experience. > > odroid u3: /dev/spidev working with good latency (good enough for 1kHz) > after kernel driver modifications, linuxcnc driver hm2_spi, mesa 7i90 > fpga card > > odroid c2: reported non-working (only supports 8-bit transfers) > > odroid xu4: does not honor spidev byte order ioctl (hardcoded in > devicetree). poor latency even after kernel driver modifications > (probably ok for 500Hz, probably not for 1kHz). hm2_spi driver again. > > raspberry pi 3: only supports 8-bit transfers. contributed > register-level driver, confirmed working on preempt-rt (only--will need > work for other supported rtoses), hm2_rpspi > > Jeff > > ------------------------------------------------------------------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, SlashDot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
