• Vladimir Oltean's avatar
    spi: spi-fsl-dspi: Always use the TCFQ devices in poll mode · 3c0f9d8b
    Vladimir Oltean authored
    With this patch, the "interrupts" property from the device tree bindings
    is ignored, even if present, if the driver runs in TCFQ mode.
    
    Switching to using the DSPI in poll mode has several distinct
    benefits:
    
    - With interrupts, the DSPI driver in TCFQ mode raises an IRQ after each
      transmitted word. There is more time wasted for the "waitq" event than
      for actual I/O. And the DSPI IRQ count can easily get the largest in
      /proc/interrupts on Freescale boards with attached SPI devices.
    
    - The SPI I/O time is both lower, and more consistently so. Attached to
      some Freescale devices are either PTP switches, or SPI RTCs. For
      reading time off of a SPI slave device, it is important that all SPI
      transfers take a deterministic time to complete.
    
    - In poll mode there is much less time spent by the CPU in hardirq
      context, which helps with the response latency of the system, and at
      the same time there is more control over when interrupts must be
      disabled (to get a precise timestamp measurement, which will come in a
      future patch): win-win.
    
    On the LS1021A-TSN board, where the SPI device is a SJA1105 PTP switch
    (with a bits_per_word=8 driver), I created a "benchmark" where I
    periodically transferred a 12-byte message once per second, for 120
    seconds. I then recorded the time before putting the first byte in the
    TX FIFO, and the time after reading the last byte from the RX FIFO. That
    is the transfer delay in nanoseconds.
    
    Interrupt mode:
    
      delay: min 125120 max 168320 mean 150286 std dev 17675.3
    
    Poll mode:
    
      delay: min 69440 max 119040 mean 70312.9 std dev 8065.34
    
    Both the mean latency and the standard deviation are more than 50% lower
    in poll mode than in interrupt mode, and the 'max' in poll mode is lower
    than the 'min' in interrupt mode. This is with an 'ondemand' governor on
    an otherwise idle system - therefore running mostly at 600 MHz out of a
    max of 1200 MHz.
    Signed-off-by: default avatarVladimir Oltean <olteanv@gmail.com>
    Link: https://lore.kernel.org/r/20191001205216.32115-1-olteanv@gmail.comSigned-off-by: default avatarMark Brown <broonie@kernel.org>
    3c0f9d8b
spi-fsl-dspi.c 29.6 KB