Re: [RFC 0/4] Improving SPI driver latency (vs v3.8.13.14-rt31)

[Jeff Epler <jepler@unpythonic.net>]

On Mon, Sep 01, 2014 at 05:41:30PM +0200, Philipp Lutz wrote:
> Have you looked into setting a RTPRIO for the SPI worker threads
> (spi_init_queue() in spi.c)? As far as I see hardly any SPI
> hardware-dependent driver sets the "rt" property in the respective
> spi_master struct, so that the main SPI driver (spi.c) will not use a RT
> priority, which will be MAX_RT_PRIO - 1.

Sort of.

I was unaware of the "rt" property so I never tried setting it on the
kernel side.  However, I did run into the idea of using "chrt" on the
kernel tasks [spi1] and [irq/99-spi-s3c6].  This improved things
somewhat, but this plus eliminating DMA acquistion still left me with
some 800+us latencies that I didn't want.

(it may have also helped somewhat to to use cpuset to bind these tasks
to the same CPU where my realtime task runs--the U3 is a 4-core chip,
and linuxcnc binds to the last CPU by default)

Jeff